 Statistics for the Behavioral Sciences, 4/e Michael Thorne,
Mississippi State University  Mississippi State Martin Giesen,
Mississippi State University  Mississippi State
Probability
Symbols and FormulasSYMBOLS Symbol  Stands 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(BA)  probability of event B given that event A has occurred 
FORMULAS Formula 71. Equation for the addition rule of probabilityp(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 72. Equation for the multiplication rule of probabilityp(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 73. Equation for determining the probability of a sequence of nonindependent eventsp(A, B) = p(A) x p(BA)
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(BA) reads "probability of B given A." 
