The number of successes in n Bernoulli trials produces a binomial random variable. The parameters of a binomial random variable are n (the number of trials) and p (the chance for success). The binomial model uses a binomial random variable to describe counts of successes observed for a real phenomenon. The use of a probability model requires assumptions about the real phenomenon, most often stability (constant probability of success) and independence. Counts based on events in disjoint intervals of time or space produce a Poisson random variable. A Poisson random variable has one parameter, its mean λ. The Poisson model uses a Poisson random variable to describe counts in data.