Poisson distribution is main concept of theoretical distribution which is invented by Simeon Denis Poisson . In this distribution some of the good points which we can not get in binomial distribution . In other worlds it birth due to some of weak points of binomial distribution . The main limitation of binomial distribution is that we can not calculate distribution if n is infinite and probability of success is very low . At these situation we can calculate Poisson distribution . Formula of calculating Poisson distribution p(r) = e's power -m multiply with m's power r and both divided by r! m = mean or average of distribution mean can be calculated = total of fx divided by total of f r = respective no. of trial e = is the base of natural logarithm = 2.7183 for more detail of probability distribution