How do I estimate the standart deviation for the mean average of an poisson-distribution ? The mean average was estimated with the maximum-likelihood method by graphing the likelihood in dependence of the mean average, then just reading off the value for which the likelihood became maximal. Up to this point I had not problem. But I also have to determine the standard deviation for my estimation of the mean average. And that is where I run into problems. I'm told that "by an deviation from (...) the mean average of the standart deviation the -2*ln(L) function increases by one unit compared with the minimum". If I understand that correctly: Minimum of the log-likelihood is 100 at an mean average of a. At an mean average of b my log-likelihood has the value 101(99). So my standart deviation is a-b. Is this interpretation correct ?