In probability theory, to say that two events are independent intuitively means that the occurrence of one event makes it neither more nor less probable that the other occurs. For example:
- The event of getting a 6 the first time a die is rolled and the event of getting a 6 the second time are independent.
- By contrast, the event of getting a 6 the first time a die is rolled and the event that the sum of the numbers seen on the first and second trials is 8 are dependent.
- If two cards are drawn with replacement from a deck of cards, the event of drawing a red card on the first trial and that of drawing a red card on the second trial are independent.
- By contrast, if two cards are drawn without replacement from a deck of cards, the event of drawing a red card on the first trial and that of drawing a red card on the second trial are dependent.
Similarly, two random variables are independent if the conditional probability distribution of either given the observed value of the other is the same as if the other's value had not been observed. The concept of independence extends to dealing with collections of more than two events or random variables.
In some instances, the term "independent" is replaced by "statistically independent", "marginally independent", or "absolutely independent".
The standard definition says:
Here A ∩ B is the intersection of A and B, that is, it is the event that both events A and B occur.
More generally, any collection of events—possibly more than just two of them—are mutually independent if and only if for every finite subset A1, ..., An of the collection we have
This is called the multiplication rule for independent events. Notice that independence requires this rule to hold for every subset of the collection; see for a three-event example in which and yet no two of the three events are pairwise independent.
If two events A and B are independent, then the conditional probability of A given B is the same as the unconditional (or marginal) probability of A, that is,
There are at least two reasons why this statement is not taken to be the definition of independence: (1) the two events A and B do not play symmetrical roles in this statement, and (2) problems arise with this statement when events of probability 0 are involved.
The conditional probability of event A given B is given by
The statement above, when is equivalent to
which is the standard definition given above.
Note that an event is independent of itself if and only if
That is, if its probability is one or zero. Thus if an event or its complement almost surely occurs, it is independent of itself. For example, if event A is choosing any number but 0.5 from a uniform distribution on the unit interval, A is independent of itself, even though, tautologically, A fully determines A.
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