Probability is the branch of mathematics and statistics concerning events and numerical descriptions of how likely they are to occur. The probability of an event is a number between 0 and 1; the larger the probability, the more likely an event is to occur. This number is often expressed as a percentage (%), ranging from 0% to 100%. A simple example is the tossing of a fair (unbiased) coin. Since the coin is fair, the two outcomes ("heads" and "tails") are both equally probable; the probability of "heads" equals the probability of "tails"; and since no other outcomes are possible, the probability of either "heads" or "tails" is 1/2 (which could also be written as 0.5 or 50%).
These concepts have been given an axiomatic mathematical formalization in probability theory, which is used widely in areas of study such as statistics, mathematics, science, finance, gambling, artificial intelligence, machine learning, computer science, game theory, and philosophy to, for example, draw inferences about the expected frequency of events. Probability theory is also used to describe the underlying mechanics and regularities of complex systems.
Etymology
The word probability derives from the Latin probabilitas, which can also mean "probity", a measure of the authority of a witness in a legal case in Europe, and often correlated with the witness's nobility. In a sense, this differs much from the modern meaning of probability, which in contrast is a measure of the weight of empirical evidence, and is arrived at from inductive reasoning and statistical inference.
Interpretations
When dealing with random experiments – i.e., experiments that are random and well-defined – in a purely theoretical setting (like tossing a coin), probabilities can be numerically described by the number of desired outcomes, divided by the total number of all outcomes. This is referred to as theoretical probability (in contrast to empirical probability, dealing with probabilities in the context of real experiments). For example, tossing a coin twice will yield "head-head", "head-tail", "tail-head", and "tail-tail" outcomes. The probability of getting an outcome of "head-head" is 1 out of 4 outcomes, or, in numerical terms, 1/4, 0.25 or 25%. However, when it comes to practical application, there are two major competing categories of probability interpretations, whose adherents hold different views about the fundamental nature of probability:
- Objectivists assign numbers to describe some objective or physical state of affairs. The most popular version of objective probability is frequentist probability, which claims that the probability of a random event denotes the relative frequency of occurrence of an experiment's outcome when the experiment is repeated indefinitely. This interpretation considers probability to be the relative frequency "in the long run" of outcomes. A modification of this is propensity probability, which interprets probability as the tendency of some experiment to yield a certain outcome, even if it is performed only once.
- Subjectivists assign numbers per subjective probability, that is, as a degree of belief. The degree of belief has been interpreted as "the price at which you would buy or sell a bet that pays 1 unit of utility if E, 0 if not E", although that interpretation is not universally agreed upon. The most popular version of subjective probability is Bayesian probability, which includes expert knowledge as well as experimental data to produce probabilities. The expert knowledge is represented by some (subjective) prior probability distribution. These data are incorporated in a likelihood function. The product of the prior and the likelihood, when normalized, results in a posterior probability distribution that incorporates all the information known to date. By Aumann's agreement theorem, Bayesian agents whose prior beliefs are similar will end up with similar posterior beliefs. However, sufficiently different priors can lead to different conclusions, regardless of how much information the agents share.
History
The scientific study of probability is a modern development of mathematics. Gambling shows that there has been an interest in quantifying the ideas of probability throughout history, but exact mathematical descriptions arose much later. There are reasons for the slow development of the mathematics of probability. Whereas games of chance provided the impetus for the mathematical study of probability, fundamental issues are still obscured by superstitions.
According to Richard Jeffrey, "Before the middle of the seventeenth century, the term 'probable' (Latin probabilis) meant approvable, and was applied in that sense, univocally, to opinion and to action. A probable action or opinion was one such as sensible people would undertake or hold, in the circumstances." However, in legal contexts especially, 'probable' could also apply to propositions for which there was good evidence.
The sixteenth-century Italian polymath Gerolamo Cardano demonstrated the efficacy of defining odds as the ratio of favourable to unfavourable outcomes (which implies that the probability of an event is given by the ratio of favourable outcomes to the total number of possible outcomes). Aside from the elementary work by Cardano, the doctrine of probabilities dates to the correspondence of Pierre de Fermat and Blaise Pascal (1654). Christiaan Huygens (1657) gave the earliest known scientific treatment of the subject.Jakob Bernoulli's Ars Conjectandi (posthumous, 1713) and Abraham de Moivre's Doctrine of Chances (1718) treated the subject as a branch of mathematics. See Ian Hacking's The Emergence of Probability and James Franklin's The Science of Conjecture for histories of the early development of the very concept of mathematical probability.
The theory of errors may be traced back to Roger Cotes's Opera Miscellanea (posthumous, 1722), but a memoir prepared by Thomas Simpson in 1755 (printed 1756) first applied the theory to the discussion of errors of observation. The reprint (1757) of this memoir lays down the axioms that positive and negative errors are equally probable, and that certain assignable limits define the range of all errors. Simpson also discusses continuous errors and describes a probability curve.
The first two laws of error that were proposed both originated with Pierre-Simon Laplace. The first law was published in 1774, and stated that the frequency of an error could be expressed as an exponential function of the numerical magnitude of the error – disregarding sign. The second law of error was proposed in 1778 by Laplace, and stated that the frequency of the error is an exponential function of the square of the error. The second law of error is called the normal distribution or the Gauss law. "It is difficult historically to attribute that law to Gauss, who in spite of his well-known precocity had probably not made this discovery before he was two years old."
Daniel Bernoulli (1778) introduced the principle of the maximum product of the probabilities of a system of concurrent errors.
Adrien-Marie Legendre (1805) developed the method of least squares, and introduced it in his Nouvelles méthodes pour la détermination des orbites des comètes (New Methods for Determining the Orbits of Comets). In ignorance of Legendre's contribution, an Irish-American writer, Robert Adrain, editor of "The Analyst" (1808), first deduced the law of facility of error,
where is a constant depending on precision of observation, and is a scale factor ensuring that the area under the curve equals 1. He gave two proofs, the second being essentially the same as John Herschel's (1850).[citation needed]Gauss gave the first proof that seems to have been known in Europe (the third after Adrain's) in 1809. Further proofs were given by Laplace (1810, 1812), Gauss (1823), James Ivory (1825, 1826), Hagen (1837), Friedrich Bessel (1838), W.F. Donkin (1844, 1856), and Morgan Crofton (1870). Other contributors were Ellis (1844), De Morgan (1864), Glaisher (1872), and Giovanni Schiaparelli (1875). Peters's (1856) formula[clarification needed] for r, the probable error of a single observation, is well known.
In the nineteenth century, authors on the general theory included Laplace, Sylvestre Lacroix (1816), Littrow (1833), Adolphe Quetelet (1853), Richard Dedekind (1860), Helmert (1872), Hermann Laurent (1873), Liagre, Didion and Karl Pearson. Augustus De Morgan and George Boole improved the exposition of the theory.
In 1906, Andrey Markov introduced the notion of Markov chains, which played an important role in stochastic processes theory and its applications. The modern theory of probability based on measure theory was developed by Andrey Kolmogorov in 1931.
On the geometric side, contributors to The Educational Times included Miller, Crofton, McColl, Wolstenholme, Watson, and Artemas Martin. See integral geometry for more information.
Theory
Like other theories, the theory of probability is a representation of its concepts in formal terms – that is, in terms that can be considered separately from their meaning. These formal terms are manipulated by the rules of mathematics and logic, and any results are interpreted or translated back into the problem domain.
There have been at least two successful attempts to formalize probability, namely the Kolmogorov formulation and the Cox formulation. In Kolmogorov's formulation (see also probability space), sets are interpreted as events and probability as a measure on a class of sets. In Cox's theorem, probability is taken as a primitive (i.e., not further analyzed), and the emphasis is on constructing a consistent assignment of probability values to propositions. In both cases, the laws of probability are the same, except for technical details.
There are other methods for quantifying uncertainty, such as the Dempster–Shafer theory or possibility theory, but those are essentially different and not compatible with the usually-understood laws of probability.
Applications
Probability theory is applied in everyday life in risk assessment and modeling. The insurance industry and markets use actuarial science to determine pricing and make trading decisions. Governments apply probabilistic methods in environmental regulation, entitlement analysis, and financial regulation.
An example of the use of probability theory in equity trading is the effect of the perceived probability of any widespread Middle East conflict on oil prices, which have ripple effects in the economy as a whole. An assessment by a commodity trader that a war is more likely can send that commodity's prices up or down, and signals other traders of that opinion. Accordingly, the probabilities are neither assessed independently nor necessarily rationally. The theory of behavioral finance emerged to describe the effect of such groupthink on pricing, on policy, and on peace and conflict.
In addition to financial assessment, probability can be used to analyze trends in biology (e.g., disease spread) as well as ecology (e.g., biological Punnett squares). As with finance, risk assessment can be used as a statistical tool to calculate the likelihood of undesirable events occurring, and can assist with implementing protocols to avoid encountering such circumstances. Probability is used to design games of chance so that casinos can make a guaranteed profit, yet provide payouts to players that are frequent enough to encourage continued play.
Another significant application of probability theory in everyday life is reliability. Many consumer products, such as automobiles and consumer electronics, use reliability theory in product design to reduce the probability of failure. Failure probability may influence a manufacturer's decisions on a product's warranty.
The cache language model and other statistical language models that are used in natural language processing are also examples of applications of probability theory.
Mathematical treatment
Consider an experiment that can produce a number of results. The collection of all possible results is called the sample space of the experiment, sometimes denoted as . The power set of the sample space is formed by considering all different collections of possible results. For example, rolling a die can produce six possible results. One collection of possible results gives an odd number on the die. Thus, the subset {1,3,5} is an element of the power set of the sample space of dice rolls. These collections are called "events". In this case, {1,3,5} is the event that the die falls on some odd number. If the results that actually occur fall in a given event, the event is said to have occurred.
A probability is a way of assigning every event a value between zero and one, with the requirement that the event made up of all possible results (in our example, the event {1,2,3,4,5,6}) is assigned a value of one. To qualify as a probability, the assignment of values must satisfy the requirement that for any collection of mutually exclusive events (events with no common results, such as the events {1,6}, {3}, and {2,4}), the probability that at least one of the events will occur is given by the sum of the probabilities of all the individual events.
The probability of an event A is written as ,, or . This mathematical definition of probability can extend to infinite sample spaces, and even uncountable sample spaces, using the concept of a measure.
The opposite or complement of an event A is the event [not A] (that is, the event of A not occurring), often denoted as , , or ; its probability is given by P(not A) = 1 − P(A). As an example, the chance of not rolling a six on a six-sided die is 1 – (chance of rolling a six) = 1 − 1/6 = 5/6. For a more comprehensive treatment, see Complementary event.
If two events A and B occur on a single performance of an experiment, this is called the intersection or joint probability of A and B, denoted as
Independent events
If two events, A and B are independent then the joint probability is
For example, if two coins are flipped, then the chance of both being heads is
Mutually exclusive events
If either event A or event B can occur but never both simultaneously, then they are called mutually exclusive events.
If two events are mutually exclusive, then the probability of both occurring is denoted as andIf two events are mutually exclusive, then the probability of either occurring is denoted as and
For example, the chance of rolling a 1 or 2 on a six-sided die is
Not (necessarily) mutually exclusive events
If the events are not (necessarily) mutually exclusive thenRewritten,
For example, when drawing a card from a deck of cards, the chance of getting a heart or a face card (J, Q, K) (or both) is since among the 52 cards of a deck, 13 are hearts, 12 are face cards, and 3 are both: here the possibilities included in the "3 that are both" are included in each of the "13 hearts" and the "12 face cards", but should only be counted once.
This can be expanded further for multiple not (necessarily) mutually exclusive events. For three events, this proceeds as follows:It can be seen, then, that this pattern can be repeated for any number of events.
Conditional probability
Conditional probability is the probability of some event A, given the occurrence of some other event B. Conditional probability is written , and is read "the probability of A, given B". It is defined by
If then is formally undefined by this expression. In this case and are independent, since However, it is possible to define a conditional probability for some zero-probability events, for example by using a σ-algebra of such events (such as those arising from a continuous random variable).
For example, in a bag of 2 red balls and 2 blue balls (4 balls in total), the probability of taking a red ball is however, when taking a second ball, the probability of it being either a red ball or a blue ball depends on the ball previously taken. For example, if a red ball was taken, then the probability of picking a red ball again would be since only 1 red and 2 blue balls would have been remaining. And if a blue ball was taken previously, the probability of taking a red ball will be
Inverse probability
In probability theory and applications, Bayes' rule relates the odds of event to event before (prior to) and after (posterior to) conditioning on another event The odds on to event is simply the ratio of the probabilities of the two events. When arbitrarily many events are of interest, not just two, the rule can be rephrased as posterior is proportional to prior times likelihood, where the proportionality symbol means that the left hand side is proportional to (i.e., equals a constant times) the right hand side as varies, for fixed or given (Lee, 2012; Bertsch McGrayne, 2012). In this form it goes back to Laplace (1774) and to Cournot (1843); see Fienberg (2005).
Summary of probabilities
Event | Probability |
---|---|
A | |
not A | |
A or B | |
A and B | |
A given B |
Relation to randomness and probability in quantum mechanics
In a deterministic universe, based on Newtonian concepts, there would be no probability if all conditions were known (Laplace's demon) (but there are situations in which sensitivity to initial conditions exceeds our ability to measure them, i.e. know them). In the case of a roulette wheel, if the force of the hand and the period of that force are known, the number on which the ball will stop would be a certainty (though as a practical matter, this would likely be true only of a roulette wheel that had not been exactly levelled – as Thomas A. Bass' Newtonian Casino revealed). This also assumes knowledge of inertia and friction of the wheel, weight, smoothness, and roundness of the ball, variations in hand speed during the turning, and so forth. A probabilistic description can thus be more useful than Newtonian mechanics for analyzing the pattern of outcomes of repeated rolls of a roulette wheel. Physicists face the same situation in the kinetic theory of gases, where the system, while deterministic in principle, is so complex (with the number of molecules typically the order of magnitude of the Avogadro constant 6.02×1023) that only a statistical description of its properties is feasible.
Probability theory is required to describe quantum phenomena. A revolutionary discovery of early 20th century physics was the random character of all physical processes that occur at sub-atomic scales and are governed by the laws of quantum mechanics. The objective wave function evolves deterministically but, according to the Copenhagen interpretation, it deals with probabilities of observing, the outcome being explained by a wave function collapse when an observation is made. However, the loss of determinism for the sake of instrumentalism did not meet with universal approval. Albert Einstein famously remarked in a letter to Max Born: "I am convinced that God does not play dice". Like Einstein, Erwin Schrödinger, who discovered the wave function, believed quantum mechanics is a statistical approximation of an underlying deterministic reality. In some modern interpretations of the statistical mechanics of measurement, quantum decoherence is invoked to account for the appearance of subjectively probabilistic experimental outcomes.
See also
- Contingency
- Equiprobability
- Fuzzy logic
- Heuristic (psychology)
Notes
- Strictly speaking, a probability of 0 indicates that an event almost never takes place, whereas a probability of 1 indicates than an event almost certainly takes place. This is an important distinction when the sample space is infinite. For example, for the continuous uniform distribution on the real interval [5, 10], there are an infinite number of possible outcomes, and the probability of any given outcome being observed — for instance, exactly 7 — is 0. This means that an observation will almost surely not be exactly 7. However, it does not mean that exactly 7 is impossible. Ultimately some specific outcome (with probability 0) will be observed, and one possibility for that specific outcome is exactly 7.
- In the context of the book that this is quoted from, it is the theory of probability and the logic behind it that governs the phenomena of such things compared to rash predictions that rely on pure luck or mythological arguments such as gods of luck helping the winner of the game.
References
- "Kendall's Advanced Theory of Statistics, Volume 1: Distribution Theory", Alan Stuart and Keith Ord, 6th ed., (2009), ISBN 978-0-534-24312-8.
- William Feller, An Introduction to Probability Theory and Its Applications, vol. 1, 3rd ed., (1968), Wiley, ISBN 0-471-25708-7.
- Probability Theory. The Britannica website.
- Hacking, I. (2006) The Emergence of Probability: A Philosophical Study of Early Ideas about Probability, Induction and Statistical Inference, Cambridge University Press, ISBN 978-0-521-68557-3 [page needed]
- Hacking, Ian (1965). The Logic of Statistical Inference. Cambridge University Press. ISBN 978-0-521-05165-1.[page needed]
- Finetti, Bruno de (1970). "Logical foundations and measurement of subjective probability". Acta Psychologica. 34: 129–145. doi:10.1016/0001-6918(70)90012-0.
- Hájek, Alan (21 October 2002). Edward N. Zalta (ed.). "Interpretations of Probability". The Stanford Encyclopedia of Philosophy (Winter 2012 ed.). Retrieved 22 April 2013.
- Jaynes, E.T. (2003). "Section A.2 The de Finetti system of probability". In Bretthorst, G. Larry (ed.). Probability Theory: The Logic of Science (1 ed.). Cambridge University Press. ISBN 978-0-521-59271-0.
- Hogg, Robert V.; Craig, Allen; McKean, Joseph W. (2004). Introduction to Mathematical Statistics (6th ed.). Upper Saddle River: Pearson. ISBN 978-0-13-008507-8.[page needed]
- Jaynes, E.T. (2003). "Section 5.3 Converging and diverging views". In Bretthorst, G. Larry (ed.). Probability Theory: The Logic of Science (1 ed.). Cambridge University Press. ISBN 978-0-521-59271-0.
- Freund, John. (1973) Introduction to Probability. Dickenson ISBN 978-0-8221-0078-2 (p. 1)
- Jeffrey, R.C., Probability and the Art of Judgment, Cambridge University Press. (1992). pp. 54–55 . ISBN 0-521-39459-7
- Franklin, J. (2001) The Science of Conjecture: Evidence and Probability Before Pascal, Johns Hopkins University Press. (pp. 22, 113, 127)
- "Some laws and problems in classical probability and how Cardano anticipated them Gorrochum, P. Chance magazine 2012" (PDF).
- Abrams, William. A Brief History of Probability. Second Moment. Archived from the original on 24 July 2017. Retrieved 23 May 2008.
- Ivancevic, Vladimir G.; Ivancevic, Tijana T. (2008). Quantum leap : from Dirac and Feynman, across the universe, to human body and mind. Singapore; Hackensack, NJ: World Scientific. p. 16. ISBN 978-981-281-927-7.
- Franklin, James (2001). The Science of Conjecture: Evidence and Probability Before Pascal. Johns Hopkins University Press. ISBN 978-0-8018-6569-5.
- Shoesmith, Eddie (November 1985). "Thomas Simpson and the arithmetic mean". Historia Mathematica. 12 (4): 352–355. doi:10.1016/0315-0860(85)90044-8.
- Wilson EB (1923) "First and second laws of error". Journal of the American Statistical Association, 18, 143
- Seneta, Eugene William. ""Adrien-Marie Legendre" (version 9)". StatProb: The Encyclopedia Sponsored by Statistics and Probability Societies. Archived from the original on 3 February 2016. Retrieved 27 January 2016.
- Weber, Richard. "Markov Chains" (PDF). Statistical Laboratory. University of Cambridge.
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{{cite book}}
: CS1 maint: location missing publisher (link) - Singh, Laurie (2010) "Whither Efficient Markets? Efficient Market Theory and Behavioral Finance". The Finance Professionals' Post, 2010.
- Edwards, Anthony William Fairbank (September 2012). "Reginald Crundall Punnett: First Arthur Balfour Professor of Genetics, Cambridge, 1912". Perspectives. Genetics. 192 (1). Gonville and Caius College, Cambridge, UK: Genetics Society of America: 3–13. doi:10.1534/genetics.112.143552. PMC 3430543. PMID 22964834. pp. 5–6:
[…] Punnett's square seems to have been a development of 1905, too late for the first edition of his Mendelism (May 1905) but much in evidence in Report III to the Evolution Committee of the Royal Society [(Bateson et al. 1906b) "received March 16, 1906"]. The earliest mention is contained in a letter to Bateson from Francis Galton dated October 1, 1905 (Edwards 2012). We have the testimony of Bateson (1909, p. 57) that "For the introduction of this system [the 'graphic method'], which greatly simplifies difficult cases, I am indebted to Mr. Punnett." […] The first published diagrams appeared in 1906. […] when Punnett published the second edition of his Mendelism, he used a slightly different format ([…] Punnett 1907, p. 45) […] In the third edition (Punnett 1911, p. 34) he reverted to the arrangement […] with a description of the construction of what he called the "chessboard" method (although in truth it is more like a multiplication table). […]
(11 pages) - Gao, J.Z.; Fong, D.; Liu, X. (April 2011). "Mathematical analyses of casino rebate systems for VIP gambling". International Gambling Studies. 11 (1): 93–106. doi:10.1080/14459795.2011.552575. S2CID 144540412.
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- Ross, Sheldon M. (2010). A First course in Probability (8th ed.). Pearson Prentice Hall. pp. 26–27. ISBN 9780136033134.
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- Jedenfalls bin ich überzeugt, daß der Alte nicht würfelt. Letter to Max Born, 4 December 1926, in: Einstein/Born Briefwechsel 1916–1955.
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Bibliography
- Kallenberg, O. (2005) Probabilistic Symmetries and Invariance Principles. Springer-Verlag, New York. 510 pp. ISBN 0-387-25115-4
- Kallenberg, O. (2002) Foundations of Modern Probability, 2nd ed. Springer Series in Statistics. 650 pp. ISBN 0-387-95313-2
- Olofsson, Peter (2005) Probability, Statistics, and Stochastic Processes, Wiley-Interscience. 504 pp ISBN 0-471-67969-0.
External links
- Virtual Laboratories in Probability and Statistics (Univ. of Ala.-Huntsville)
- Probability on In Our Time at the BBC
- Probability and Statistics EBook
- Edwin Thompson Jaynes. Probability Theory: The Logic of Science. Preprint: Washington University, (1996). – HTML index with links to PostScript files and PDF (first three chapters)
- People from the History of Probability and Statistics (Univ. of Southampton)
- Probability and Statistics on the Earliest Uses Pages (Univ. of Southampton)
- Earliest Uses of Symbols in Probability and Statistics on Earliest Uses of Various Mathematical Symbols
- A tutorial on probability and Bayes' theorem devised for first-year Oxford University students
- U B U W E B :: La Monte Young pdf file of An Anthology of Chance Operations (1963) at UbuWeb
- Introduction to Probability – eBook Archived 27 July 2011 at the Wayback Machine, by Charles Grinstead, Laurie Snell Source Archived 25 March 2012 at the Wayback Machine (GNU Free Documentation License)
- (in English and Italian) Bruno de Finetti, Probabilità e induzione, Bologna, CLUEB, 1993. ISBN 88-8091-176-7 (digital version)
- Richard Feynman's Lecture on probability.
Probability is the branch of mathematics and statistics concerning events and numerical descriptions of how likely they are to occur The probability of an event is a number between 0 and 1 the larger the probability the more likely an event is to occur This number is often expressed as a percentage ranging from 0 to 100 A simple example is the tossing of a fair unbiased coin Since the coin is fair the two outcomes heads and tails are both equally probable the probability of heads equals the probability of tails and since no other outcomes are possible the probability of either heads or tails is 1 2 which could also be written as 0 5 or 50 The probabilities of rolling several numbers using two dice These concepts have been given an axiomatic mathematical formalization in probability theory which is used widely in areas of study such as statistics mathematics science finance gambling artificial intelligence machine learning computer science game theory and philosophy to for example draw inferences about the expected frequency of events Probability theory is also used to describe the underlying mechanics and regularities of complex systems EtymologyThe word probability derives from the Latin probabilitas which can also mean probity a measure of the authority of a witness in a legal case in Europe and often correlated with the witness s nobility In a sense this differs much from the modern meaning of probability which in contrast is a measure of the weight of empirical evidence and is arrived at from inductive reasoning and statistical inference InterpretationsWhen dealing with random experiments i e experiments that are random and well defined in a purely theoretical setting like tossing a coin probabilities can be numerically described by the number of desired outcomes divided by the total number of all outcomes This is referred to as theoretical probability in contrast to empirical probability dealing with probabilities in the context of real experiments For example tossing a coin twice will yield head head head tail tail head and tail tail outcomes The probability of getting an outcome of head head is 1 out of 4 outcomes or in numerical terms 1 4 0 25 or 25 However when it comes to practical application there are two major competing categories of probability interpretations whose adherents hold different views about the fundamental nature of probability Objectivists assign numbers to describe some objective or physical state of affairs The most popular version of objective probability is frequentist probability which claims that the probability of a random event denotes the relative frequency of occurrence of an experiment s outcome when the experiment is repeated indefinitely This interpretation considers probability to be the relative frequency in the long run of outcomes A modification of this is propensity probability which interprets probability as the tendency of some experiment to yield a certain outcome even if it is performed only once Subjectivists assign numbers per subjective probability that is as a degree of belief The degree of belief has been interpreted as the price at which you would buy or sell a bet that pays 1 unit of utility if E 0 if not E although that interpretation is not universally agreed upon The most popular version of subjective probability is Bayesian probability which includes expert knowledge as well as experimental data to produce probabilities The expert knowledge is represented by some subjective prior probability distribution These data are incorporated in a likelihood function The product of the prior and the likelihood when normalized results in a posterior probability distribution that incorporates all the information known to date By Aumann s agreement theorem Bayesian agents whose prior beliefs are similar will end up with similar posterior beliefs However sufficiently different priors can lead to different conclusions regardless of how much information the agents share HistoryThe scientific study of probability is a modern development of mathematics Gambling shows that there has been an interest in quantifying the ideas of probability throughout history but exact mathematical descriptions arose much later There are reasons for the slow development of the mathematics of probability Whereas games of chance provided the impetus for the mathematical study of probability fundamental issues are still obscured by superstitions According to Richard Jeffrey Before the middle of the seventeenth century the term probable Latin probabilis meant approvable and was applied in that sense univocally to opinion and to action A probable action or opinion was one such as sensible people would undertake or hold in the circumstances However in legal contexts especially probable could also apply to propositions for which there was good evidence Gerolamo Cardano 16th century Christiaan Huygens published one of the first books on probability 17th century The sixteenth century Italian polymath Gerolamo Cardano demonstrated the efficacy of defining odds as the ratio of favourable to unfavourable outcomes which implies that the probability of an event is given by the ratio of favourable outcomes to the total number of possible outcomes Aside from the elementary work by Cardano the doctrine of probabilities dates to the correspondence of Pierre de Fermat and Blaise Pascal 1654 Christiaan Huygens 1657 gave the earliest known scientific treatment of the subject Jakob Bernoulli s Ars Conjectandi posthumous 1713 and Abraham de Moivre s Doctrine of Chances 1718 treated the subject as a branch of mathematics See Ian Hacking s The Emergence of Probability and James Franklin s The Science of Conjecture for histories of the early development of the very concept of mathematical probability The theory of errors may be traced back to Roger Cotes s Opera Miscellanea posthumous 1722 but a memoir prepared by Thomas Simpson in 1755 printed 1756 first applied the theory to the discussion of errors of observation The reprint 1757 of this memoir lays down the axioms that positive and negative errors are equally probable and that certain assignable limits define the range of all errors Simpson also discusses continuous errors and describes a probability curve The first two laws of error that were proposed both originated with Pierre Simon Laplace The first law was published in 1774 and stated that the frequency of an error could be expressed as an exponential function of the numerical magnitude of the error disregarding sign The second law of error was proposed in 1778 by Laplace and stated that the frequency of the error is an exponential function of the square of the error The second law of error is called the normal distribution or the Gauss law It is difficult historically to attribute that law to Gauss who in spite of his well known precocity had probably not made this discovery before he was two years old Daniel Bernoulli 1778 introduced the principle of the maximum product of the probabilities of a system of concurrent errors Carl Friedrich Gauss Adrien Marie Legendre 1805 developed the method of least squares and introduced it in his Nouvelles methodes pour la determination des orbites des cometes New Methods for Determining the Orbits of Comets In ignorance of Legendre s contribution an Irish American writer Robert Adrain editor of The Analyst 1808 first deduced the law of facility of error ϕ x ce h2x2 displaystyle phi x ce h 2 x 2 where h displaystyle h is a constant depending on precision of observation and c displaystyle c is a scale factor ensuring that the area under the curve equals 1 He gave two proofs the second being essentially the same as John Herschel s 1850 citation needed Gauss gave the first proof that seems to have been known in Europe the third after Adrain s in 1809 Further proofs were given by Laplace 1810 1812 Gauss 1823 James Ivory 1825 1826 Hagen 1837 Friedrich Bessel 1838 W F Donkin 1844 1856 and Morgan Crofton 1870 Other contributors were Ellis 1844 De Morgan 1864 Glaisher 1872 and Giovanni Schiaparelli 1875 Peters s 1856 formula clarification needed for r the probable error of a single observation is well known In the nineteenth century authors on the general theory included Laplace Sylvestre Lacroix 1816 Littrow 1833 Adolphe Quetelet 1853 Richard Dedekind 1860 Helmert 1872 Hermann Laurent 1873 Liagre Didion and Karl Pearson Augustus De Morgan and George Boole improved the exposition of the theory In 1906 Andrey Markov introduced the notion of Markov chains which played an important role in stochastic processes theory and its applications The modern theory of probability based on measure theory was developed by Andrey Kolmogorov in 1931 On the geometric side contributors to The Educational Times included Miller Crofton McColl Wolstenholme Watson and Artemas Martin See integral geometry for more information TheoryLike other theories the theory of probability is a representation of its concepts in formal terms that is in terms that can be considered separately from their meaning These formal terms are manipulated by the rules of mathematics and logic and any results are interpreted or translated back into the problem domain There have been at least two successful attempts to formalize probability namely the Kolmogorov formulation and the Cox formulation In Kolmogorov s formulation see also probability space sets are interpreted as events and probability as a measure on a class of sets In Cox s theorem probability is taken as a primitive i e not further analyzed and the emphasis is on constructing a consistent assignment of probability values to propositions In both cases the laws of probability are the same except for technical details There are other methods for quantifying uncertainty such as the Dempster Shafer theory or possibility theory but those are essentially different and not compatible with the usually understood laws of probability ApplicationsProbability theory is applied in everyday life in risk assessment and modeling The insurance industry and markets use actuarial science to determine pricing and make trading decisions Governments apply probabilistic methods in environmental regulation entitlement analysis and financial regulation An example of the use of probability theory in equity trading is the effect of the perceived probability of any widespread Middle East conflict on oil prices which have ripple effects in the economy as a whole An assessment by a commodity trader that a war is more likely can send that commodity s prices up or down and signals other traders of that opinion Accordingly the probabilities are neither assessed independently nor necessarily rationally The theory of behavioral finance emerged to describe the effect of such groupthink on pricing on policy and on peace and conflict In addition to financial assessment probability can be used to analyze trends in biology e g disease spread as well as ecology e g biological Punnett squares As with finance risk assessment can be used as a statistical tool to calculate the likelihood of undesirable events occurring and can assist with implementing protocols to avoid encountering such circumstances Probability is used to design games of chance so that casinos can make a guaranteed profit yet provide payouts to players that are frequent enough to encourage continued play Another significant application of probability theory in everyday life is reliability Many consumer products such as automobiles and consumer electronics use reliability theory in product design to reduce the probability of failure Failure probability may influence a manufacturer s decisions on a product s warranty The cache language model and other statistical language models that are used in natural language processing are also examples of applications of probability theory Mathematical treatmentCalculation of probability risk vs odds Consider an experiment that can produce a number of results The collection of all possible results is called the sample space of the experiment sometimes denoted as W displaystyle Omega The power set of the sample space is formed by considering all different collections of possible results For example rolling a die can produce six possible results One collection of possible results gives an odd number on the die Thus the subset 1 3 5 is an element of the power set of the sample space of dice rolls These collections are called events In this case 1 3 5 is the event that the die falls on some odd number If the results that actually occur fall in a given event the event is said to have occurred A probability is a way of assigning every event a value between zero and one with the requirement that the event made up of all possible results in our example the event 1 2 3 4 5 6 is assigned a value of one To qualify as a probability the assignment of values must satisfy the requirement that for any collection of mutually exclusive events events with no common results such as the events 1 6 3 and 2 4 the probability that at least one of the events will occur is given by the sum of the probabilities of all the individual events The probability of an event A is written as P A displaystyle P A p A displaystyle p A or Pr A displaystyle text Pr A This mathematical definition of probability can extend to infinite sample spaces and even uncountable sample spaces using the concept of a measure The opposite or complement of an event A is the event not A that is the event of A not occurring often denoted as A Ac displaystyle A A c A A A displaystyle overline A A complement neg A or A displaystyle sim A its probability is given by P not A 1 P A As an example the chance of not rolling a six on a six sided die is 1 chance of rolling a six 1 1 6 5 6 For a more comprehensive treatment see Complementary event If two events A and B occur on a single performance of an experiment this is called the intersection or joint probability of A and B denoted as P A B displaystyle P A cap B Independent events If two events A and B are independent then the joint probability is Events A and B depicted as independent vs non independent in space W For example if two coins are flipped then the chance of both being heads is 12 12 14 displaystyle tfrac 1 2 times tfrac 1 2 tfrac 1 4 Mutually exclusive events If either event A or event B can occur but never both simultaneously then they are called mutually exclusive events If two events are mutually exclusive then the probability of both occurring is denoted as P A B displaystyle P A cap B andP A and B P A B 0 displaystyle P A mbox and B P A cap B 0 If two events are mutually exclusive then the probability of either occurring is denoted as P A B displaystyle P A cup B andP A or B P A B P A P B P A B P A P B 0 P A P B displaystyle P A mbox or B P A cup B P A P B P A cap B P A P B 0 P A P B For example the chance of rolling a 1 or 2 on a six sided die is P 1 or 2 P 1 P 2 16 16 13 displaystyle P 1 mbox or 2 P 1 P 2 tfrac 1 6 tfrac 1 6 tfrac 1 3 Not necessarily mutually exclusive events If the events are not necessarily mutually exclusive thenP A or B P A B P A P B P A and B displaystyle P left A hbox or B right P A cup B P left A right P left B right P left A mbox and B right Rewritten P A B P A P B P A B displaystyle P left A cup B right P left A right P left B right P left A cap B right For example when drawing a card from a deck of cards the chance of getting a heart or a face card J Q K or both is 1352 1252 352 1126 displaystyle tfrac 13 52 tfrac 12 52 tfrac 3 52 tfrac 11 26 since among the 52 cards of a deck 13 are hearts 12 are face cards and 3 are both here the possibilities included in the 3 that are both are included in each of the 13 hearts and the 12 face cards but should only be counted once This can be expanded further for multiple not necessarily mutually exclusive events For three events this proceeds as follows P A B C P A B C P A B P C P A B C P A P B P A B P C P A C B C P A P B P C P A B P A C P B C P A C B C P A B C P A P B P C P A B P A C P B C P A B C displaystyle begin aligned P left A cup B cup C right amp P left left A cup B right cup C right amp P left A cup B right P left C right P left left A cup B right cap C right amp P left A right P left B right P left A cap B right P left C right P left left A cap C right cup left B cap C right right amp P left A right P left B right P left C right P left A cap B right left P left A cap C right P left B cap C right P left left A cap C right cap left B cap C right right right P left A cup B cup C right amp P left A right P left B right P left C right P left A cap B right P left A cap C right P left B cap C right P left A cap B cap C right end aligned It can be seen then that this pattern can be repeated for any number of events Conditional probability Conditional probability is the probability of some event A given the occurrence of some other event B Conditional probability is written P A B displaystyle P A mid B and is read the probability of A given B It is defined by P A B P A B P B displaystyle P A mid B frac P A cap B P B If P B 0 displaystyle P B 0 then P A B displaystyle P A mid B is formally undefined by this expression In this case A displaystyle A and B displaystyle B are independent since P A B P A P B 0 displaystyle P A cap B P A P B 0 However it is possible to define a conditional probability for some zero probability events for example by using a s algebra of such events such as those arising from a continuous random variable For example in a bag of 2 red balls and 2 blue balls 4 balls in total the probability of taking a red ball is 1 2 displaystyle 1 2 however when taking a second ball the probability of it being either a red ball or a blue ball depends on the ball previously taken For example if a red ball was taken then the probability of picking a red ball again would be 1 3 displaystyle 1 3 since only 1 red and 2 blue balls would have been remaining And if a blue ball was taken previously the probability of taking a red ball will be 2 3 displaystyle 2 3 Inverse probability In probability theory and applications Bayes rule relates the odds of event A1 displaystyle A 1 to event A2 displaystyle A 2 before prior to and after posterior to conditioning on another event B displaystyle B The odds on A1 displaystyle A 1 to event A2 displaystyle A 2 is simply the ratio of the probabilities of the two events When arbitrarily many events A displaystyle A are of interest not just two the rule can be rephrased as posterior is proportional to prior times likelihood P A B P A P B A displaystyle P A B propto P A P B A where the proportionality symbol means that the left hand side is proportional to i e equals a constant times the right hand side as A displaystyle A varies for fixed or given B displaystyle B Lee 2012 Bertsch McGrayne 2012 In this form it goes back to Laplace 1774 and to Cournot 1843 see Fienberg 2005 Summary of probabilities Summary of probabilities Event ProbabilityA P A 0 1 displaystyle P A in 0 1 not A P A 1 P A displaystyle P A complement 1 P A A or B P A B P A P B P A B P A B P A P B if A and B are mutually exclusive displaystyle begin aligned P A cup B amp P A P B P A cap B P A cup B amp P A P B qquad mbox if A and B are mutually exclusive end aligned A and B P A B P A B P B P B A P A P A B P A P B if A and B are independent displaystyle begin aligned P A cap B amp P A B P B P B A P A P A cap B amp P A P B qquad mbox if A and B are independent end aligned A given B P A B P A B P B P B A P A P B displaystyle P A mid B frac P A cap B P B frac P B A P A P B Relation to randomness and probability in quantum mechanicsIn a deterministic universe based on Newtonian concepts there would be no probability if all conditions were known Laplace s demon but there are situations in which sensitivity to initial conditions exceeds our ability to measure them i e know them In the case of a roulette wheel if the force of the hand and the period of that force are known the number on which the ball will stop would be a certainty though as a practical matter this would likely be true only of a roulette wheel that had not been exactly levelled as Thomas A Bass Newtonian Casino revealed This also assumes knowledge of inertia and friction of the wheel weight smoothness and roundness of the ball variations in hand speed during the turning and so forth A probabilistic description can thus be more useful than Newtonian mechanics for analyzing the pattern of outcomes of repeated rolls of a roulette wheel Physicists face the same situation in the kinetic theory of gases where the system while deterministic in principle is so complex with the number of molecules typically the order of magnitude of the Avogadro constant 6 02 1023 that only a statistical description of its properties is feasible Probability theory is required to describe quantum phenomena A revolutionary discovery of early 20th century physics was the random character of all physical processes that occur at sub atomic scales and are governed by the laws of quantum mechanics The objective wave function evolves deterministically but according to the Copenhagen interpretation it deals with probabilities of observing the outcome being explained by a wave function collapse when an observation is made However the loss of determinism for the sake of instrumentalism did not meet with universal approval Albert Einstein famously remarked in a letter to Max Born I am convinced that God does not play dice Like Einstein Erwin Schrodinger who discovered the wave function believed quantum mechanics is a statistical approximation of an underlying deterministic reality In some modern interpretations of the statistical mechanics of measurement quantum decoherence is invoked to account for the appearance of subjectively probabilistic experimental outcomes See alsoMathematics portalPhilosophy portal Contingency Equiprobability Fuzzy logic Heuristic psychology NotesStrictly speaking a probability of 0 indicates that an event almost never takes place whereas a probability of 1 indicates than an event almost certainly takes place This is an important distinction when the sample space is infinite For example for the continuous uniform distribution on the real interval 5 10 there are an infinite number of possible outcomes and the probability of any given outcome being observed for instance exactly 7 is 0 This means that an observation will almost surely not be exactly 7 However it does not mean that exactly 7 is impossible Ultimately some specific outcome with probability 0 will be observed and one possibility for that specific outcome is exactly 7 In the context of the book that this is quoted from it is the theory of probability and the logic behind it that governs the phenomena of such things compared to rash predictions that rely on pure luck or mythological arguments such as gods of luck helping the winner of the game References Kendall s Advanced Theory of Statistics Volume 1 Distribution Theory Alan Stuart and Keith Ord 6th ed 2009 ISBN 978 0 534 24312 8 William Feller An Introduction to Probability Theory and Its Applications vol 1 3rd ed 1968 Wiley ISBN 0 471 25708 7 Probability Theory The Britannica website Hacking I 2006 The Emergence of Probability A Philosophical Study of Early Ideas about Probability Induction and Statistical Inference Cambridge University Press ISBN 978 0 521 68557 3 page needed Hacking Ian 1965 The Logic of Statistical Inference Cambridge University Press ISBN 978 0 521 05165 1 page needed Finetti Bruno de 1970 Logical foundations and measurement of subjective probability Acta Psychologica 34 129 145 doi 10 1016 0001 6918 70 90012 0 Hajek Alan 21 October 2002 Edward N Zalta ed Interpretations of Probability The Stanford Encyclopedia of Philosophy Winter 2012 ed Retrieved 22 April 2013 Jaynes E T 2003 Section A 2 The de Finetti system of probability In Bretthorst G Larry ed Probability Theory The Logic of Science 1 ed Cambridge University Press ISBN 978 0 521 59271 0 Hogg Robert V Craig Allen McKean Joseph W 2004 Introduction to Mathematical Statistics 6th ed Upper Saddle River Pearson ISBN 978 0 13 008507 8 page needed Jaynes E T 2003 Section 5 3 Converging and diverging views In Bretthorst G Larry ed Probability Theory The Logic of Science 1 ed Cambridge University Press ISBN 978 0 521 59271 0 Freund John 1973 Introduction to Probability Dickenson ISBN 978 0 8221 0078 2 p 1 Jeffrey R C Probability and the Art of Judgment Cambridge University Press 1992 pp 54 55 ISBN 0 521 39459 7 Franklin J 2001 The Science of Conjecture Evidence and Probability Before Pascal Johns Hopkins University Press pp 22 113 127 Some laws and problems in classical probability and how Cardano anticipated them Gorrochum P Chance magazine 2012 PDF Abrams William A Brief History of Probability Second Moment Archived from the original on 24 July 2017 Retrieved 23 May 2008 Ivancevic Vladimir G Ivancevic Tijana T 2008 Quantum leap from Dirac and Feynman across the universe to human body and mind Singapore Hackensack NJ World Scientific p 16 ISBN 978 981 281 927 7 Franklin James 2001 The Science of Conjecture Evidence and Probability Before Pascal Johns Hopkins University Press ISBN 978 0 8018 6569 5 Shoesmith Eddie November 1985 Thomas Simpson and the arithmetic mean Historia Mathematica 12 4 352 355 doi 10 1016 0315 0860 85 90044 8 Wilson EB 1923 First and second laws of error Journal of the American Statistical Association 18 143 Seneta Eugene William Adrien Marie Legendre version 9 StatProb The Encyclopedia Sponsored by Statistics and Probability Societies Archived from the original on 3 February 2016 Retrieved 27 January 2016 Weber Richard Markov Chains PDF Statistical Laboratory University of Cambridge Vitanyi Paul M B 1988 Andrei Nikolaevich Kolmogorov CWI Quarterly 1 3 18 Retrieved 27 January 2016 Wilcox Rand R 2016 Understanding and applying basic statistical methods using R Hoboken New Jersey ISBN 978 1 119 06140 3 OCLC 949759319 a href wiki Template Cite book title Template Cite book cite book a CS1 maint location missing publisher link Singh Laurie 2010 Whither Efficient Markets Efficient Market Theory and Behavioral Finance The Finance Professionals Post 2010 Edwards Anthony William Fairbank September 2012 Reginald Crundall Punnett First Arthur Balfour Professor of Genetics Cambridge 1912 Perspectives Genetics 192 1 Gonville and Caius College Cambridge UK Genetics Society of America 3 13 doi 10 1534 genetics 112 143552 PMC 3430543 PMID 22964834 pp 5 6 Punnett s square seems to have been a development of 1905 too late for the first edition of his Mendelism May 1905 but much in evidence in Report III to the Evolution Committee of the Royal Society Bateson et al 1906b received March 16 1906 The earliest mention is contained in a letter to Bateson from Francis Galton dated October 1 1905 Edwards 2012 We have the testimony of Bateson 1909 p 57 that For the introduction of this system the graphic method which greatly simplifies difficult cases I am indebted to Mr Punnett The first published diagrams appeared in 1906 when Punnett published the second edition of his Mendelism he used a slightly different format Punnett 1907 p 45 In the third edition Punnett 1911 p 34 he reverted to the arrangement with a description of the construction of what he called the chessboard method although in truth it is more like a multiplication table 11 pages Gao J Z Fong D Liu X April 2011 Mathematical analyses of casino rebate systems for VIP gambling International Gambling Studies 11 1 93 106 doi 10 1080 14459795 2011 552575 S2CID 144540412 Gorman Michael F 2010 Management Insights Management Science 56 iv vii doi 10 1287 mnsc 1090 1132 Ross Sheldon M 2010 A First course in Probability 8th ed Pearson Prentice Hall pp 26 27 ISBN 9780136033134 Weisstein Eric W Probability mathworld wolfram com Retrieved 10 September 2020 Olofsson 2005 p 8 Olofsson 2005 p 9 Olofsson 2005 p 35 Olofsson 2005 p 29 Conditional probability with respect to a sigma algebra www statlect com Retrieved 4 July 2022 Riedi P C 1976 Kinetic Theory of Gases I In Thermal Physics Palgrave London https doi org 10 1007 978 1 349 15669 6 8 Burgin Mark 2010 Interpretations of Negative Probabilities p 1 arXiv 1008 1287v1 physics data an Jedenfalls bin ich uberzeugt dass der Alte nicht wurfelt Letter to Max Born 4 December 1926 in Einstein Born Briefwechsel 1916 1955 Moore W J 1992 Schrodinger Life and Thought Cambridge University Press p 479 ISBN 978 0 521 43767 7 BibliographyKallenberg O 2005 Probabilistic Symmetries and Invariance Principles Springer Verlag New York 510 pp ISBN 0 387 25115 4 Kallenberg O 2002 Foundations of Modern Probability 2nd ed Springer Series in Statistics 650 pp ISBN 0 387 95313 2 Olofsson Peter 2005 Probability Statistics and Stochastic Processes Wiley Interscience 504 pp ISBN 0 471 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Probability eBook Archived 27 July 2011 at the Wayback Machine by Charles Grinstead Laurie Snell Source Archived 25 March 2012 at the Wayback Machine GNU Free Documentation License in English and Italian Bruno de Finetti Probabilita e induzione Bologna CLUEB 1993 ISBN 88 8091 176 7 digital version Richard Feynman s Lecture on probability Portal Mathematics