Figure 5. Probability density function. Graph shows Poisson probability density functions for three values of µ (mu). The y axis reflects the probability of obtaining a measurement that has the specific value listed on the x axis. For µ = 2, the probability distribution is asymmetric (skewed) with maxima at k = 1 and k = 2 yielding the same probability of 28%. As µ increases, the distributions become broader, more symmetric, and bell shaped and the probability of observing one specific number of events is reduced because the data are more widely dispersed.