p-value The probability, expressed as a number, that a particular effect or association is real or that a given statement or hypothesis is true. If a trial has n possible outcomes and m of these are the desired outcome, then the probability (p) of obtaining the desired outcome is m /n . If all the outcomes are as desired then m = n and p = 1. If none are as desired p = 0. So p is a measure that will alwys range from 0 to 1.