I'm dealing with a histogram and want to use poisson errors for each bin. For example, having 7 items in a bin gives that bin an error of sqrt(7). I'm comparing four different data sets, each with different sizes. I'm scaling everything in terms of probabilities so the four data sets can be compared. My smallest data set contains only 8 elements, of which 7 are in one bin, with an error of sqrt(7). Now when this is scaled to a probability, y=7/8 and yerr=sqrt(7)/8. However, this gives P=0.875+-0.33, which does not make sence since the probability cannot exceed 0. Is there something simple I'm missing?