multinomial distribution

multinomial distribution, in statistics, a generalization of the binomial distribution, which admits only two values (such as success and failure), to more than two values. Like the binomial distribution, the multinomial distribution is a distribution function for discrete processes in which fixed probabilities prevail for each independently generated value. Although processes involving multinomial distributions can be studied using the binomial distribution by focusing on one result of interest and combining all of the other results into one category (simplifying the distribution to two values), multinomial distributions are more useful when all of the results are of interest.

Multinomial distributions are common in biological and geological applications. For example, 19th-century Austrian botanist Gregor Mendel crossed two strains of peas, one with green and wrinkled seeds and one with yellow and smooth seeds, which produced strains with four different seeds: green and wrinkled, yellow and round, green and round, and yellow and wrinkled. His study of the resulting multinomial distribution led him to discover the basic principles of genetics.

In symbols, a multinomial distribution involves a process that has a set of k possible results (X₁, X₂, X₃,…, X_k) with associated probabilities (p₁, p₂, p₃,…, p_k) such that Σp_i = 1. The sum of the probabilities must equal 1 because one of the results is sure to occur. Then for n repeated trials of the process, let x_i indicate the number of times that the result X_i occurs, subject to the restraints that 0 ≤ x_i ≤ n and Σx_i = n. With this notation, the joint probability density function is given by