## Quick Reference

The discrete probability distribution for the number of experiments required to achieve the first success in a sequence of independent experiments, all with the same probability *p* of success. The probability mass function is given by Pr(*X*=*r*)=*p*(1−*p*)^{r−1}, for *r*=1, 2,…. It has mean 1/*p* and variance (1−*p*)/*p*^{2}.

*Subjects:*
Probability and Statistics.

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