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Probability is the branch of mathematics concerning numerical descriptions of how likely an event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and 1, where, roughly speaking, 0 indicates impossibility of the event and 1 indicates certainty. 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 when we make an observation, it 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. The higher the probability of an event, the more likely it is that the event will occur.

These concepts have been given an axiomatic mathematical formalization in probability theory (see probability axioms), which is used widely in such areas of study as mathematics, statistics, finance, gambling, science (in particular physics), 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.

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. The power set of the sample space is formed by considering all different collections of possible results. For example, rolling a dice can produce six possible results. One collection of possible results gives an odd number on the dice. 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 dice 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 if you look at a collection of mutually exclusive events (events with no common results, e.g., the events {1,6}, {3}, and {2,4} are all mutually exclusive), 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 [math]A[/math] is written as [math]\operatorname{P}(A)[/math] or [math]\text{Pr}(A)[/math]. 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 [math]A[/math] is the event [not [math]A[/math]] (that is, the event of [math]A[/math] not occurring), often denoted as [math]\overline{A}, A^C, \neg A[/math], or [math]\sim A[/math]; its probability is given by [math]\operatorname{P}[/math](not [math]A[/math]) = 1 − [math]\operatorname{P}[/math]([math]A[/math]). As an example, the chance of not rolling a six on a six-sided die is 1 – (chance of rolling a six) [math]= 1 - \tfrac{1}{6} = \tfrac{5}{6}[/math]. See Complementary event for a more complete treatment.

If two events [math]A[/math] and [math]B[/math] occur on a single performance of an experiment, this is called the intersection of [math]A[/math] and [math]B[/math], denoted as [math]\operatorname{P}(A \cap B)[/math].

Independent events

If two events, [math]A[/math] and [math]B[/math] are independent then the joint probability is

[[math]]\operatorname{P}(A \mbox{ and }B) = \operatorname{P}(A \cap B) = \operatorname{P}(A) \operatorname{P}(B),\,[[/math]]

for example, if two coins are flipped the chance of both being heads is [math]\tfrac{1}{2}\times\tfrac{1}{2} = \tfrac{1}{4}[/math].

Mutually exclusive events

If either event [math]A[/math] or event [math]B[/math] occurs on a single performance of an experiment this is called the union of the events [math]A[/math] and [math]B[/math] denoted as [math]\operatorname{P}(A \cup B)[/math]. If two events are mutually exclusive then the probability of either occurring is

[[math]]\operatorname{P}(A\mbox{ or }B) = \operatorname{P}(A \cup B)= \operatorname{P}(A) + \operatorname{P}(B).[[/math]]

For example, the chance of rolling a 1 or 2 on a six-sided dice is [math]\operatorname{P}(1\mbox{ or }2) = \operatorname{P}(1) + \operatorname{P}(2) = \tfrac{1}{6} + \tfrac{1}{6} = \tfrac{1}{3}.[/math]

Not mutually exclusive events

If the events are not mutually exclusive then

[[math]]\operatorname{P}\left( A \hbox{ or } B\right)=\operatorname{P}\left( A\right)+\operatorname{P}\left( B\right)-\operatorname{P}\left( A \cap B\right).[[/math]]

For example, when drawing a single card at random from a regular deck of cards, the chance of getting a heart or a face card (J,Q,K) (or one that is both) is [math]\tfrac{13}{52} + \tfrac{12}{52} - \tfrac{3}{52} = \tfrac{11}{26}[/math], because of 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.

Conditional probability

Conditional probability is the probability of some event [math]A[/math], given the occurrence of some other event [math]B[/math]. Conditional probability is written [math]\operatorname{P}(A \mid B)[/math], and is read "the probability of [math]A[/math], given [math]B[/math]". It is defined by

[[math]]\operatorname{P}(A \mid B) = \frac{\operatorname{P}(A \cap B)}{\operatorname{P}(B)}.\,[[/math]]

If [math]\operatorname{P}(B)=0[/math] then [math]\operatorname{P}(A \mid B)[/math] is formally undefined by this expression. However, it is possible to define a conditional probability for some zero-probability events 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 [math]1/2[/math]; 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, such as, if a red ball was taken, the probability of picking a red ball again would be [math]1/3[/math] since only 1 red and 2 blue balls would have been remaining.

Summary of probabilities

Event	Probability
[math]A[/math]	[math]\operatorname{P}(A)\in[0,1]\,[/math]
not [math]A[/math]	[math]\operatorname{P}(A^c)=1-\operatorname{P}(A)\,[/math]
[math]A[/math] or [math]B[/math]	[math] \operatorname{P}(A\cup B) = \operatorname{P}(A)+\operatorname{P}(B)-\operatorname{P}(A\cap B) [/math]
[math]A[/math] and [math]B[/math]	[math] \operatorname{P}(A\cap B) = \operatorname{P}(A|B)\operatorname{P}(B) = \operatorname{P}(B|A)\operatorname{P}(A) [/math]
[math]A[/math] given [math]B[/math]	[math]\operatorname{P}(A \mid B) = \frac{\operatorname{P}(A \cap B)}{\operatorname{P}(B)} = \frac{\operatorname{P}(B|A)\operatorname{P}(A)}{\operatorname{P}(B)} \,[/math]

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.
• Wikipedia contributors. "Probability". Wikipedia. Wikipedia. Retrieved 28 January 2022.