... other, and vice versa peA or B) = peA) + PCB) - peA and B) if A and B are disjoint, peA or B) = P(A) + PCB) Subtract peA and B) so as not to count twice the elements of both A and B peA and B) ... ("expectation") and standard deviation of random variables; if we can char acterize a random variable as belonging to some major family (see table below), we can find the mean and standard deviation ... into several disjoint and exhaustive events 0• \', 2, 3, , ~ , then, since A must occur along With one and only one of the D's: peA) = peA and 0,) + P(A and O2) + + peA and OJ = P(D,)P(AID,)...