What is a cumulative probability?
A cumulative probability is a sum of probabilities. In connection with the normal distribution, a cumulative probability refers to the probability that a randomly selected score will be less than or equal to a specified value, referred to as the normal random variable. Suppose, for example, that we have a school with 100 first-graders. If we ask about the probability that a randomly selected first grader weighs exactly 70 pounds, we are asking about a simple probability – not a cumulative probability. But if we ask about the probability that a randomly selected first grader is less than or equal to 70 pounds, we are really asking about a sum of probabilities (i.e., the probability that the student is exactly 70 pounds plus the probability that he/she is 69 pounds plus the probability that he/she is 68 pounds, etc.). Thus, we are asking about a cumulative probability.
