Mixtures

Contents - Index

Mixture Distributions

Mixture distributions are used to model individual heterogeneity of capture probabilities (Pledger 1999, Norris and Pollock 1996) in the closed captures data type. To change the number of mixture distributions, you have to change data type, available under the PIM menu choice. Mixture models have also been added for the occupancy data type, and the robust design occupancy data types.

Mixture models consist of a pi parameter that gives the probability of a mixture, with the sum of the pi values equaling 1. In MARK, 1 less pi value is included in the model, so that the last pi value is gotten by substraction. When you specify >2 mixtures, the multinomial logit link function (mlogit link function) is useful to maintain this constraint.

Mixture models can have multiple modes, so care with their optimization should be taken. The simulated annealing algorithm included in MARK is a good method to avoid local maxima.