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**Statistics:**
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**Probability theory** is the mathematical study of phenomena characterizedby randomness or uncertainty.

More precisely, **probability** is used for modelling situations when the result of an experiment, realized under the same circumstances, produces different results (typically throwing a dice or a coin).
Mathematicians and think of probabilities as numbers in the closed interval from 0 to 1 assigned to "events" whose occurrence or failure to occur is random. Probabilities are assigned to events according to the probability axioms.

The probability that an event occurs *given* the known occurrence of an event is the conditional probability of *given* ; its numerical value is (as long as is nonzero). If the conditional probability of given is the same as the ("unconditional") probability of , then and are said to be independent events. That this relation between and is symmetric may be seen more readily by realizing that it is the same as saying
when A and B are independent events.

Two crucial concepts in the theory of probability are those of a random variable and of the probability distribution of a random variable; see those articles for more information.

## A somewhat more abstract view of probability

Mathematicians usually take probability theory to be the study of probability spaces and random variables — an approach introduced by Kolmogorov in the 1930s. A probability space is a triple , where

- is a non-empty set, sometimes called the "sample space," each of whose members is thought of as a potential outcome of a random experiment. For example, if 100 voters are to be drawn randomly from among all voters in California and asked whom they will vote for governor, then the set of all sequences of 100 Californian voters would be the sample space Ω.

- is a σ-algebra of subsets of - its members are called "events." For example the set of all sequences of 100 Californian voters in which at least 60 will vote for Schwarzenegger is identified with the "event" that at least 60 of the 100 chosen voters will so vote. To say that is a σ-algebra implies per definition that it contains , that the complement of any event is an event, and that the union of any (finite or countably infinite) sequence of events is an event. So for this example contains: (1) the set of all sequences of 100 where at least 60 vote for Schwarzenegger; (2) the set of all sequences of 100 where fewer than 60 vote for Schwarzenegger (the converse of (1)); (3) the sample space Ω as above; and (4) the empty set.

- is a probability measure on , i.e., a measure such that .

It is important to note that is a function defined on and not on , and often not on the complete powerset either. Not every set of outcomes is an event.

If is denumerable we almost always define as the power set of , i.e which is trivially a σ-algebra and the biggest one we can create using . In a discrete space we can therefore omit and just write to define it. If on the other hand is non-denumerable and we use we get into trouble defining our probability measure because is too 'huge', i.e. there will often be sets to which it will be impossible to assign a unique measure, giving rise to problems like the Banach–Tarski paradox. So we have to use a smaller σ-algebra (e.g. the Borel algebra of , which is the smallest σ-algebra that makes all open sets measurable).

A random variable is a measurable function on . For example, the number of voters who will vote for Schwarzenegger in the aforementioned sample of 100 is a random variable.

If is any random variable, the notation , is shorthand for , assuming that "" is an "event."

For an algebraic alternative to Kolmogorov's approach, see algebra of random variables.

## Philosophy of application of probability

There are different ways to interpret probability. Frequentists will assign probabilities only to *events* that are *random*, i.e., random variables, that are outcomes of actual or theoretical *experiments*. On the other hand, Bayesians assign probabilities to *propositions* that are *uncertain* according either to subjective degrees of belief in their truth, or to logically justifiable degrees of belief in their truth. Among statisticians and philosophers, many more distinctions are drawn beyond this subjective/objective divide. See the article on interpretations of probability at the Stanford Encyclopedia of Philosophy: [1].

A Bayesian may assign a probability to the proposition that 'there was life on Mars a billion years ago', since that is uncertain, whereas a frequentist would not assign probabilities to *statements* at all. A frequentist is actually unable to technically interpret such uses of the probability concept, even though 'probability' is often used in this way in colloquial speech. Frequentists only assign probabilities to outcomes of well defined *random experiments*, that is, where there is a defined sample space as defined above in the theory section. For another illustration of the differences see the two envelopes problem.

Situations do arise where probability theory is somewhat lacking. One method of attempting to circumvent this indeterminancy is the theory of super-probability, in which situations are given integer values greater than 1.

## See also

- Probability axioms
- probabilistic logic
- credence (probability theory)
- glossary of probability and statistics
- list of probability topics
- list of statistical topics
- list of publications in statistics
- predictive modelling
- fuzzy measure theory
- probability axioms
- probability distribution
- expected value
- likelihood function
- random variable
- sample space
- variance
- statistical independence
- notation in probability
- possibility theory
- Pierre de Fermat
- Blaise Pascal

## Bibliography

- 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:

- Andrei Nikolajevich Kolmogorov (1950)
*Foundations of the Theory of Probability*

- The modern measure-theoretic foundation of probability theory; the original German version (
*Grundbegriffe der Wahrscheinlichkeitrechnung*) appeared in 1933.

- The modern measure-theoretic foundation of probability theory; the original German version (

- Harold Jeffreys (1939)
*The Theory of Probability*

- An empiricist, Bayesian approach to the foundations of probability theory.

- Edward Nelson (1987)
*Radically Elementary Probability Theory*

- Discrete foundations of probability theory, based on nonstandard analysis and internal set theory. downloadable. http://www.math.princeton.edu/~nelson/books.html

- Patrick Billingsley:
*Probability and Measure*, John Wiley and Sons, New York, Toronto, London, 1979.

- Henk Tijms (2004)
*Understanding Probability*

- A lively introduction to probability theory for the beginner, Cambridge Univ. Press.

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