1. A method of assembling a cohort for a client being one of a population, comprising testing whether models related to the population excluding the client meet an acceptance threshold test as to identity with a model for the client, determining, from each model meeting the threshold test, whether those models are distributed so as to present at least a given probability that models for non-members of the population spaced from the client model in all directions will each be closer to a member of the cohort, excluding the client, than to the client and, if that probability is less than a predetermined value, selecting from the population another cohort member which will reduce that probability.