The Skellam distribution is the discrete probability distribution of the difference N 1 − N 2 {\displaystyle N_{1}-N_{2}} of two statistically independent random variables N 1 {\displaystyle N_{1}} and N 2, {\displaystyle N_{2},} each Poisson-distributed with respective expected values μ 1 {\displaystyle \mu _{1}} and μ 2 {\displaystyle \mu _{2}}. Wikipedia