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**Probability theory** is the branch of mathematics concerned with probability. Although there are several different probability interpretations, probability theory treats the concept in a rigorous mathematical manner by expressing it through a set of axioms. Typically these axioms formalise probability in terms of a probability space, which assigns a measure taking values between 0 and 1, termed the probability measure, to a set of outcomes called the sample space. Any specified subset of these outcomes is called an event. Central subjects in probability theory include discrete and continuous random variables, probability distributions, and stochastic processes, which provide mathematical abstractions of non-deterministic or uncertain processes or measured quantities that may either be single occurrences or evolve over time in a random fashion. Although it is not possible to perfectly predict random events, much can be said about their behavior. Two major results in probability theory describing such behaviour are the law of large numbers and the central limit theorem.

- History of probability
- Treatment
- Motivation
- Discrete probability distributions
- Continuous probability distributions
- Measure-theoretic probability theory
- Classical probability distributions
- Convergence of random variables
- Law of large numbers
- Central limit theorem
- See also
- Notes
- References

As a mathematical foundation for statistics, probability theory is essential to many human activities that involve quantitative analysis of data.^{ [1] } Methods of probability theory also apply to descriptions of complex systems given only partial knowledge of their state, as in statistical mechanics or sequential estimation. A great discovery of twentieth-century physics was the probabilistic nature of physical phenomena at atomic scales, described in quantum mechanics.^{ [2] }

The early form of statistical inference were developed by Arab mathematicians studying cryptography between the 8th and 13th centuries. Al-Khalil (717–786) wrote the *Book of Cryptographic Messages*, which contains the first use of permutations and combinations to list all possible Arabic words with and without vowels. Al-Kindi (801–873) made the earliest known use of statistical inference in his work on cryptanalysis and frequency analysis. An important contribution of Ibn Adlan (1187–1268) was on sample size for use of frequency analysis.^{ [3] }

The modern mathematical theory of probability has its roots in attempts to analyze games of chance by Gerolamo Cardano in the sixteenth century, and by Pierre de Fermat and Blaise Pascal in the seventeenth century (for example the "problem of points"). Christiaan Huygens published a book on the subject in 1657^{ [4] } and in the 19th century, Pierre Laplace completed what is today considered the classic interpretation.^{ [5] }

Initially, probability theory mainly considered *discrete* events, and its methods were mainly combinatorial. Eventually, analytical considerations compelled the incorporation of *continuous* variables into the theory.

This culminated in modern probability theory, on foundations laid by Andrey Nikolaevich Kolmogorov. Kolmogorov combined the notion of sample space, introduced by Richard von Mises, and measure theory and presented his axiom system for probability theory in 1933. This became the mostly undisputed axiomatic basis for modern probability theory; but, alternatives exist, such as the adoption of finite rather than countable additivity by Bruno de Finetti.^{ [6] }

Most introductions to probability theory treat discrete probability distributions and continuous probability distributions separately. The measure theory-based treatment of probability covers the discrete, continuous, a mix of the two, and more.

Consider an experiment that can produce a number of outcomes. The set of all outcomes is called the * sample space * of the experiment. The * power set * of the sample space (or equivalently, the event space) is formed by considering all different collections of possible results. For example, rolling an honest die produces one of six possible results. One collection of possible results corresponds to getting an odd number. Thus, the subset {1,3,5} is an element of the power set of the sample space of die 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, that event is said to have occurred.

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}) be assigned a value of one. To qualify as a probability distribution, the assignment of values must satisfy the requirement that if you look at a collection of mutually exclusive events (events that contain no common results, e.g., the events {1,6}, {3}, and {2,4} are all mutually exclusive), the probability that any of these events occurs is given by the sum of the probabilities of the events.^{ [7] }

The probability that any one of the events {1,6}, {3}, or {2,4} will occur is 5/6. This is the same as saying that the probability of event {1,2,3,4,6} is 5/6. This event encompasses the possibility of any number except five being rolled. The mutually exclusive event {5} has a probability of 1/6, and the event {1,2,3,4,5,6} has a probability of 1, that is, absolute certainty.

When doing calculations using the outcomes of an experiment, it is necessary that all those elementary events have a number assigned to them. This is done using a random variable. A random variable is a function that assigns to each elementary event in the sample space a real number. This function is usually denoted by a capital letter.^{ [8] } In the case of a die, the assignment of a number to a certain elementary events can be done using the identity function. This does not always work. For example, when flipping a coin the two possible outcomes are "heads" and "tails". In this example, the random variable *X* could assign to the outcome "heads" the number "0" () and to the outcome "tails" the number "1" ().

*Discrete probability theory* deals with events that occur in countable sample spaces.

Examples: Throwing dice, experiments with decks of cards, random walk, and tossing coins

*Classical definition*: Initially the probability of an event to occur was defined as the number of cases favorable for the event, over the number of total outcomes possible in an equiprobable sample space: see Classical definition of probability.

For example, if the event is "occurrence of an even number when a die is rolled", the probability is given by , since 3 faces out of the 6 have even numbers and each face has the same probability of appearing.

*Modern definition*: The modern definition starts with a finite or countable set called the sample space, which relates to the set of all *possible outcomes* in classical sense, denoted by . It is then assumed that for each element , an intrinsic "probability" value is attached, which satisfies the following properties:

That is, the probability function *f*(*x*) lies between zero and one for every value of *x* in the sample space *Ω*, and the sum of *f*(*x*) over all values *x* in the sample space *Ω* is equal to 1. An * event * is defined as any subset of the sample space . The *probability* of the event is defined as

So, the probability of the entire sample space is 1, and the probability of the null event is 0.

The function mapping a point in the sample space to the "probability" value is called a *probability mass function* abbreviated as *pmf*. The modern definition does not try to answer how probability mass functions are obtained; instead, it builds a theory that assumes their existence^{[ citation needed ]}.

*Continuous probability theory* deals with events that occur in a continuous sample space.

*Classical definition*: The classical definition breaks down when confronted with the continuous case. See Bertrand's paradox.

*Modern definition*: If the outcome space of a random variable *X* is the set of real numbers () or a subset thereof, then a function called the * cumulative distribution function * (or *cdf*) exists, defined by . That is, *F*(*x*) returns the probability that *X* will be less than or equal to *x*.

The cdf necessarily satisfies the following properties.

- is a monotonically non-decreasing, right-continuous function;

If is absolutely continuous, i.e., its derivative exists and integrating the derivative gives us the cdf back again, then the random variable *X* is said to have a * probability density function * or *pdf* or simply *density*

For a set , the probability of the random variable *X* being in is

In case the probability density function exists, this can be written as

Whereas the *pdf* exists only for continuous random variables, the *cdf* exists for all random variables (including discrete random variables) that take values in

These concepts can be generalized for multidimensional cases on and other continuous sample spaces.

The * raison d'être * of the measure-theoretic treatment of probability is that it unifies the discrete and the continuous cases, and makes the difference a question of which measure is used. Furthermore, it covers distributions that are neither discrete nor continuous nor mixtures of the two.

An example of such distributions could be a mix of discrete and continuous distributions—for example, a random variable that is 0 with probability 1/2, and takes a random value from a normal distribution with probability 1/2. It can still be studied to some extent by considering it to have a pdf of , where is the Dirac delta function.

Other distributions may not even be a mix, for example, the Cantor distribution has no positive probability for any single point, neither does it have a density. The modern approach to probability theory solves these problems using measure theory to define the probability space:

Given any set (also called *sample space*) and a σ-algebra on it, a measure defined on is called a *probability measure* if

If is the Borel σ-algebra on the set of real numbers, then there is a unique probability measure on for any cdf, and vice versa. The measure corresponding to a cdf is said to be *induced* by the cdf. This measure coincides with the pmf for discrete variables and pdf for continuous variables, making the measure-theoretic approach free of fallacies.

The *probability* of a set in the σ-algebra is defined as

where the integration is with respect to the measure induced by

Along with providing better understanding and unification of discrete and continuous probabilities, measure-theoretic treatment also allows us to work on probabilities outside , as in the theory of stochastic processes. For example, to study Brownian motion, probability is defined on a space of functions.

When it's convenient to work with a dominating measure, the Radon-Nikodym theorem is used to define a density as the Radon-Nikodym derivative of the probability distribution of interest with respect to this dominating measure. Discrete densities are usually defined as this derivative with respect to a counting measure over the set of all possible outcomes. Densities for absolutely continuous distributions are usually defined as this derivative with respect to the Lebesgue measure. If a theorem can be proved in this general setting, it holds for both discrete and continuous distributions as well as others; separate proofs are not required for discrete and continuous distributions.

Certain random variables occur very often in probability theory because they well describe many natural or physical processes. Their distributions, therefore, have gained *special importance* in probability theory. Some fundamental *discrete distributions* are the discrete uniform, Bernoulli, binomial, negative binomial, Poisson and geometric distributions. Important *continuous distributions* include the continuous uniform, normal, exponential, gamma and beta distributions.

In probability theory, there are several notions of convergence for random variables. They are listed below in the order of strength, i.e., any subsequent notion of convergence in the list implies convergence according to all of the preceding notions.

- Weak convergence
- A sequence of random variables converges
*weakly*to the random variable if their respective cumulative*distribution functions*converge to the cumulative distribution function of , wherever is continuous. Weak convergence is also called*convergence in distribution*.

- Most common shorthand notation:

- Convergence in probability
- The sequence of random variables is said to converge towards the random variable
*in probability*if for every ε > 0.

- Most common shorthand notation:

- Strong convergence
- The sequence of random variables is said to converge towards the random variable
*strongly*if . Strong convergence is also known as*almost sure convergence*.

- Most common shorthand notation:

As the names indicate, weak convergence is weaker than strong convergence. In fact, strong convergence implies convergence in probability, and convergence in probability implies weak convergence. The reverse statements are not always true.

Common intuition suggests that if a fair coin is tossed many times, then *roughly* half of the time it will turn up *heads*, and the other half it will turn up *tails*. Furthermore, the more often the coin is tossed, the more likely it should be that the ratio of the number of *heads* to the number of *tails* will approach unity. Modern probability theory provides a formal version of this intuitive idea, known as the *law of large numbers*. This law is remarkable because it is not assumed in the foundations of probability theory, but instead emerges from these foundations as a theorem. Since it links theoretically derived probabilities to their actual frequency of occurrence in the real world, the law of large numbers is considered as a pillar in the history of statistical theory and has had widespread influence.^{ [9] }

The *law of large numbers* (LLN) states that the sample average

of a sequence of independent and identically distributed random variables converges towards their common expectation , provided that the expectation of is finite.

It is in the different forms of convergence of random variables that separates the *weak* and the *strong* law of large numbers

- Weak law: for

- Strong law: for

It follows from the LLN that if an event of probability *p* is observed repeatedly during independent experiments, the ratio of the observed frequency of that event to the total number of repetitions converges towards *p*.

For example, if are independent Bernoulli random variables taking values 1 with probability *p* and 0 with probability 1-*p*, then for all *i*, so that converges to *p* almost surely.

"The central limit theorem (CLT) is one of the great results of mathematics." (Chapter 18 in^{ [10] }) It explains the ubiquitous occurrence of the normal distribution in nature.

The theorem states that the average of many independent and identically distributed random variables with finite variance tends towards a normal distribution *irrespective* of the distribution followed by the original random variables. Formally, let be independent random variables with mean and variance Then the sequence of random variables

converges in distribution to a standard normal random variable.

For some classes of random variables the classic central limit theorem works rather fast (see Berry–Esseen theorem), for example the distributions with finite first, second, and third moment from the exponential family; on the other hand, for some random variables of the heavy tail and fat tail variety, it works very slowly or may not work at all: in such cases one may use the Generalized Central Limit Theorem (GCLT).

- Catalog of articles in probability theory
- Expected value and Variance
- Fuzzy logic and Fuzzy measure theory
- Glossary of probability and statistics
- Likelihood function
- List of probability topics
- List of publications in statistics
- List of statistical topics
- Notation in probability
- Predictive modelling
- Probabilistic logic – A combination of probability theory and logic
- Probabilistic proofs of non-probabilistic theorems
- Probability distribution
- Probability axioms
- Probability interpretations
- Probability space
- Statistical independence
- Statistical physics
- Subjective logic
- Probability of the union of pairwise independent events

- ↑ Inferring From Data
- ↑ "Why is quantum mechanics based on probability theory?".
*StackExchange*. July 1, 2014.^{[ unreliable source? ]} - ↑ Broemeling, Lyle D. (1 November 2011). "An Account of Early Statistical Inference in Arab Cryptology".
*The American Statistician*.**65**(4): 255–257. doi:10.1198/tas.2011.10191. - ↑ Grinstead, Charles Miller; James Laurie Snell. "Introduction".
*Introduction to Probability*. pp. vii. - ↑ Hájek, Alan. "Interpretations of Probability" . Retrieved 2012-06-20.Unknown parameter
`|book-title=`

ignored (help) - ↑ ""The origins and legacy of Kolmogorov's Grundbegriffe", by Glenn Shafer and Vladimir Vovk" (PDF). Retrieved 2012-02-12.
- ↑ Ross, Sheldon (2010).
*A First Course in Probability*(8th ed.). Pearson Prentice Hall. pp. 26–27. ISBN 978-0-13-603313-4 . Retrieved 2016-02-28. - ↑ Bain, Lee J.; Engelhardt, Max (1992).
*Introduction to Probability and Mathematical Statistics*(2nd ed.). Belmont, California: Brooks/Cole. p. 53. ISBN 978-0-534-38020-5. - ↑ "Leithner & Co Pty Ltd - Value Investing, Risk and Risk Management - Part I". Leithner.com.au. 2000-09-15. Archived from the original on 2014-01-26. Retrieved 2012-02-12.
- ↑ David Williams, "Probability with martingales", Cambridge 1991/2008

In probability theory, the **expected value** of a random variable , denoted or , is a generalization of the weighted average, and is intuitively the arithmetic mean of a large number of independent realizations of . The expected value is also known as the **expectation**, **mathematical expectation**, **mean**, **average**, or **first moment**. Expected value is a key concept in economics, finance, and many other subjects.

In probability theory and statistics, a **probability distribution** is the mathematical function that gives the probabilities of occurrence of different possible **outcomes** for an experiment. It is a mathematical description of a random phenomenon in terms of its sample space and the probabilities of events.

In probability and statistics, a **random variable**, **random quantity**, **aleatory variable**, or **stochastic variable** is described informally as a variable whose values depend on outcomes of a random phenomenon. The formal mathematical treatment of random variables is a topic in probability theory. In that context, a random variable is understood as a measurable function defined on a probability space that maps from the sample space to the real numbers.

In probability theory, a **probability space** or a **probability triple** is a mathematical construct that provides a formal model of a random process or "experiment". For example, one can define a probability space which models the throwing of a die.

In probability theory and related fields, a **stochastic** or **random process** is a mathematical object usually defined as a family of random variables. Stochastic processes are widely used as mathematical models of systems and phenomena that appear to vary in a random manner. Examples include the growth of a bacterial population, an electrical current fluctuating due to thermal noise, or the movement of a gas molecule. Stochastic processes have applications in many disciplines such as biology, chemistry, ecology, neuroscience, physics, image processing, signal processing, control theory, information theory, computer science, cryptography and telecommunications. Furthermore, seemingly random changes in financial markets have motivated the extensive use of stochastic processes in finance.

In probability theory, there exist several different notions of **convergence of random variables**. The convergence of sequences of random variables to some limit random variable is an important concept in probability theory, and its applications to statistics and stochastic processes. The same concepts are known in more general mathematics as **stochastic convergence** and they formalize the idea that a sequence of essentially random or unpredictable events can sometimes be expected to settle down into a behavior that is essentially unchanging when items far enough into the sequence are studied. The different possible notions of convergence relate to how such a behavior can be characterized: two readily understood behaviors are that the sequence eventually takes a constant value, and that values in the sequence continue to change but can be described by an unchanging probability distribution.

In mathematics, a **degenerate distribution** is a probability distribution in a space with support only on a space of lower dimension. If the degenerate distribution is univariate it is a **deterministic distribution** and takes only a single value. Examples include a two-headed coin and rolling a die whose sides all show the same number. This distribution satisfies the definition of "random variable" even though it does not appear random in the everyday sense of the word; hence it is considered degenerate.

In probability and statistics, a **probability mass function** (**PMF**) is a function that gives the probability that a discrete random variable is exactly equal to some value. Sometimes it is also known as the discrete density function. The probability mass function is often the primary means of defining a discrete probability distribution, and such functions exist for either scalar or multivariate random variables whose domain is discrete.

In information theory, the **asymptotic equipartition property** (**AEP**) is a general property of the output samples of a stochastic source. It is fundamental to the concept of typical set used in theories of data compression.

In probability theory, an event is said to happen **almost surely** if it happens with probability 1. In other words, the set of possible exceptions may be non-empty, but it has probability 0. The concept is analogous to the concept of "almost everywhere" in measure theory.

In probability theory and statistics, given two jointly distributed random variables and , the **conditional probability distribution** of *Y* given *X* is the probability distribution of when is known to be a particular value; in some cases the conditional probabilities may be expressed as functions containing the unspecified value of as a parameter. When both and are categorical variables, a conditional probability table is typically used to represent the conditional probability. The conditional distribution contrasts with the marginal distribution of a random variable, which is its distribution without reference to the value of the other variable.

Probability theory and statistics have some commonly used conventions, in addition to standard mathematical notation and mathematical symbols.

In probability theory, **random element** is a generalization of the concept of random variable to more complicated spaces than the simple real line. The concept was introduced by Maurice Fréchet (1948) who commented that the “development of probability theory and expansion of area of its applications have led to necessity to pass from schemes where (random) outcomes of experiments can be described by number or a finite set of numbers, to schemes where outcomes of experiments represent, for example, vectors, functions, processes, fields, series, transformations, and also sets or collections of sets.”

In mathematics, **uniform integrability** is an important concept in real analysis, functional analysis and measure theory, and plays a vital role in the theory of martingales. The definition used in measure theory is closely related to, but not identical to, the definition typically used in probability.

In probability theory, a **standard probability space**, also called **Lebesgue–Rokhlin probability space** or just **Lebesgue space** is a probability space satisfying certain assumptions introduced by Vladimir Rokhlin in 1940. Informally, it is a probability space consisting of an interval and/or a finite or countable number of atoms.

In mathematics – specifically, in the theory of stochastic processes – **Doob's martingale convergence theorems** are a collection of results on the limits of supermartingales, named after the American mathematician Joseph L. Doob. Informally, the **martingale convergence theorem** typically refers to the result that any supermartingale satisfying a certain boundedness condition must converge. One may think of supermartingales as the random variable analogues of non-increasing sequences; from this perspective, the martingale convergence theorem is a random variable analogue of the monotone convergence theorem, which states that any bounded monotone sequence converges. There are symmetric results for submartingales, which are analogous to non-decreasing sequences.

In probability theory, a **continuous stochastic process** is a type of stochastic process that may be said to be "continuous" as a function of its "time" or index parameter. Continuity is a nice property for a process to have, since it implies that they are well-behaved in some sense, and, therefore, much easier to analyze. It is implicit here that the index of the stochastic process is a continuous variable. Some authors define a "continuous (stochastic) process" as only requiring that the index variable be continuous, without continuity of sample paths: in some terminology, this would be a continuous-time stochastic process, in parallel to a "discrete-time process". Given the possible confusion, caution is needed.

In probability theory, **regular conditional probability** is a concept that formalizes the notion of conditioning on the outcome of a random variable. The resulting **conditional probability distribution** is a parametrized family of probability measures called a Markov kernel.

In probability theory, the **continuous mapping theorem** states that continuous functions preserve limits even if their arguments are sequences of random variables. A continuous function, in Heine’s definition, is such a function that maps convergent sequences into convergent sequences: if *x _{n}* →

In probability theory, **conditional probability** is a measure of the probability of an event occurring, given that another event has already occurred. If the event of interest is A and the event B is known or assumed to have occurred, "the conditional probability of A given B", or "the probability of A under the condition B", is usually written as P(*A*|*B*), or sometimes P_{B}(*A*) or P(*A*/*B*). For example, the probability that any given person has a cough on any given day may be only 5%. But if we know or assume that the person is sick, then they are much more likely to be coughing. For example, the conditional probability that someone unwell is coughing might be 75%, in which case we would have that P(Cough) = 5% and P(Cough|Sick) = 75%.

This article includes a list of general references, but it remains largely unverified because it lacks sufficient corresponding inline citations .(September 2009) |

- Pierre Simon de Laplace (1812).
*Analytical Theory of Probability*.

- The first major treatise blending calculus with probability theory, originally in French:
*Théorie Analytique des Probabilités*.

- The first major treatise blending calculus with probability theory, originally in French:

- A. Kolmogoroff (1933).
*Grundbegriffe der Wahrscheinlichkeitsrechnung*. doi:10.1007/978-3-642-49888-6. ISBN 978-3-642-49888-6.

- An English translation by Nathan Morrison appeared under the title
*Foundations of the Theory of Probability*(Chelsea, New York) in 1950, with a second edition in 1956.

- An English translation by Nathan Morrison appeared under the title

- Patrick Billingsley (1979).
*Probability and Measure*. New York, Toronto, London: John Wiley and Sons. - Olav Kallenberg;
*Foundations of Modern Probability,*2nd ed. Springer Series in Statistics. (2002). 650 pp. ISBN 0-387-95313-2 - Henk Tijms (2004).
*Understanding Probability*. Cambridge Univ. Press.

- A lively introduction to probability theory for the beginner.

- Olav Kallenberg;
*Probabilistic Symmetries and Invariance Principles*. Springer -Verlag, New York (2005). 510 pp. ISBN 0-387-25115-4 - Gut, Allan (2005).
*Probability: A Graduate Course*. Springer-Verlag. ISBN 0-387-22833-0.

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