A Bayesian Study of the Multinomial Distribution

Lindley [6] studies the topic in our title. By using Fisher's conditional-Poisson approach to the multinomial and the logarithmic transformation of gamma variables to normality, he showed that linear contrasts in the logarithms of the cell probabilities θᵢ are asymptotically jointly normal and suggested that the approximation can be improved by applying a "correction" to the sample. By studying the asymptotic series for the joint distribution in Section 2 an improved correction procedure is found below. A more detailed expansion is given in Section 3 for the distribution of a single contrast in the log θᵢ. In many problems a linear function of the θᵢ is of interest. The exact distribution is obtained and is of a form familiar in the theory of serial correlation coefficients. A beta approximation is given. For three cells, a numerical example is given to show the merit of this approximation. A genetic linkage example is considered which requires the joint distribution of two linear functions of the θᵢ. The exact joint distribution is found but is too involved for practical use. A normal approximation leads to Lindley's results [7].