I need to fit a Poisson distribution to this set of data (no. of counts of radioactive decay) The number of counts in a fixed time interval was recorded 500 times. With the number of counts going from 0 - 9 respectively 39 106 130 100 67 34 15 7 1 1 I understand how to use the formula in simple probability situations but am a little stuck with this one. I know the mean = the variance and worked that out to be 67.1 but when using this in the formlua I calculated explodes (shows math error). Can someone help me please?