We characterize probabilistic choice models that have two (or more) fixed ratio scales
associated with each choice option. A version of Luce's choice model with fixed scales and
exponents, which we also characterize, is included. The characterizing properties are:
Simple scalability, ie the choice probabilities depend upon a priori arbitrary combinations of
the two scale values attached to each choice option. Aggregation equations express that
both the probabilities and the scales are aggregated from individual to overall values�…