POrSSON'S PROBABILITY SUMMATION 



605 



stituting the sample in question to be repeated an infinite number of 

 times, independenth' and uniformly, with an average number of 

 occurrences per sample equal to a, so that we may speak of a as the 

 average number of occurrences for the sample in question. The 



iin{lili iili[iil||i|il{lilili 



3 4 3 6 7 



Values of a 



Fig. 1 — Probability curves showing Poisson's exponential summation 



for the probability P that an event occur at least c times in a large group of trials 



for which the average number of occurrences is a. A scale proportional to the normal 



probability integral is used for P, a linear scale for a 



