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Probability

The likelihood of the occurrence of an event. The probability of event A is written P(A). Probabilities are always numbers between 0 and 1, inclusive.

The four basic rules of probability:

1. For any event A, 0 ≤ P(A) ≤ 1.

2. P(impossible event) = 0.
Also written P(empty set) = 0 or P() = 0.

3. P(sure event) = 1.
Also written P(S) = 1, where S is the sample space.

4. P(not A) = 1 – P(A).
Also written P(complement of A) = 1 – P(A)   or   P(AC) = 1 – P(A)   or   .

If all outcomes of an experiment are equally likely, then

 

See also

Addition rule, multiplication rule, odds

 


  this page updated 21-feb-16
Mathwords: Terms and Formulas from Algebra I to Calculus
written, illustrated, and webmastered by Bruce Simmons
NCTM Web Bytes December 2004 Web Bytes March 2005 Web Bytes