Predators sometimes eat disproportionately more of the abundant forms of their prey, promoting diversity in the prey population. Such selection may be investigated by exposing a series of prey populations with different relative frequencies of the various types to a series of predators. A simple model is used to describe the outcome of such a series of trials, relating the relative frequencies of the various prey types eaten to the relative frequencies available. Frequency-independent and frequency-dependent components of selection are separated in the model. Previous criticism of the model and the validity of the new model are discussed. Two techniques for fitting the model to the data from experiments with 2 prey types are described. An appropriate technique for fitting this model to the data from experiments with more than 2 prey types is given.