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Thorne and Giesen Book Cover
Statistics for the Behavioral Sciences, 4/e
Michael Thorne, Mississippi State University -- Mississippi State
Martin Giesen, Mississippi State University -- Mississippi State


Symbols and Formulas


    SymbolStands For

    p(A) probability of event A
    p(A or B) probability of event A or event B
    p(A, B) probability of both A and B
    p(B|A) probability of event B given that event A has occurred


Formula 7-1. Equation for the addition rule of probability

    p(A or B) = p(A) + p(B)

    p(A or B) means the probability of either event A or event B, and it is equal to the probability of event A
    [p(A)] plus the probability of event B [p(B)].

Formula 7-2. Equation for the multiplication rule of probability

    p(A, B) = p(A) x p(B)

    p(A, B) is the probability of occurrence of both event A and event B, which is equal to the product of their individual probabilities. This equation is used when events A and B are independent.

Formula 7-3. Equation for determining the probability of a sequence of nonindependent events

    p(A, B) = p(A) x p(B|A)

    When events A and B are not independent—that is, when the probability of B depends on whether A has occurred—then the multiplication rule must be modified as shown. p(B|A) reads "probability of B given A."