Probability space
Mathematical model of random processes and experiments.
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A probability space, or probability triple, is a mathematical framework used to formally model a random process or experiment. It is built from three components: a sample space, an event space, and a probability function. These components must follow a set of rules known as the probability axioms. The sample space is the set of all possible outcomes for the random process being studied.
An outcome is the result of a single run of the model, and every run must produce exactly one outcome. The event space is a collection of events, where each event is a subset of the sample space (containing zero or more outcomes). When an experiment is performed, the resulting outcome determines which events have occurred: any event that contains that outcome is said to have happened.
The probability function assigns a number between 0 and 1 to each event in the event space. This number represents the event's probability. If the experiment were repeated many times, the fraction of trials in which an event occurs would tend to approach its assigned probability.
For example, consider a probability space modeling a single die throw. The sample space is typically the set {1, 2, 3, 4, 5, 6}, where each element labels a possible outcome. The event space could be all subsets of this sample space, including simple events like {"the die lands on 5"} and complex events like {"the die lands on an even number"}. The probability function would then map each event to the number of outcomes it contains divided by 6.
Quick Facts
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Lore & Background
A probability space is a mathematical triplet (Ω, F, P) that formally models a random process or experiment. The sample space, Ω, is a non-empty set containing every possible outcome of the process; each execution of the experiment yields exactly one outcome from this set. The event space, F, is a σ-algebra—a collection of subsets of Ω that includes the sample space itself, is closed under complementation, and is closed under countable unions.
Events are these subsets, and an event is said to have occurred if the outcome of an experiment is an element of that event. The probability function, P, is a set function that assigns to each event a real number between 0 and 1. It must satisfy two axioms: the probability of a countable union of mutually exclusive events equals the countable sum of their individual probabilities, and the probability of the entire sample space is exactly 1. For example, in a model of a single die throw, Ω is typically the set {1,2,3,4,5,6}; F could be the set of all subsets of Ω, including simple events like {5} and complex events like {2,4,6}; and P assigns to each event its size divided by 6.
The Soviet mathematician Andrey Kolmogorov introduced this notion and the associated axioms in the 1930s. In modern probability theory, alternative axiomatizations exist, such as the algebra of random variables. Not every subset of Ω need be an event; some subsets may be unmeasurable or of no interest, as in models involving continuous measurements like javelin throw lengths, where events are typically intervals and their unions.
Reader's Guide
The concept of a probability space was introduced by the Soviet mathematician Andrey Kolmogorov in the 1930s. It serves as the foundation for modern probability theory, allowing precise modeling of random experiments.
For example, a die throw can be modeled with sample space {1,2,3,4,5,6}, an event space of all subsets, and a probability function assigning each event a probability equal to the number of outcomes in that event divided by 6. The framework ensures that when an experiment is repeated many times, the relative frequency of each event tends toward its assigned probability. Alternative axiomatizations exist, such as the algebra of random variables.
Frequently Asked Questions
Who is Probability space?
Probability space is the foundational formal framework in probability theory that gives a rigorous mathematical structure to any random experiment or stochastic process. It is also commonly referred to as a probability triple.
What are the three components of Probability space?
The triple consists of a sample space (the full set of elementary outcomes), an event space (a collection of subsets of the sample space to which probabilities can be assigned), and a probability function (a measure mapping events to values between zero and one while obeying Kolmogorov's axioms).
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Probability space (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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