 Constrained Poisson and multinomial likelihood equivalence

For cell means ${\mu }_j$ with ${\sum }_j{\mu }_j=n$, the independent Poisson distribution likelihood and the multinomial distribution likelihood with probabilities ${\mu }_j/n$ differ by a parameter-independent factor. Thus a maximum-likelihood estimator for the unrestricted Poisson model whose fitted total is $n$ is also an estimator for the constrained multinomial model. Equality of named parameter estimates presumes identifiability and uniqueness; without these, the conclusion concerns the sets of maximizers. A free intercept in a log-linear Poisson model enforces the fitted-total equation.