A fully Bayesian method for quantitative genetic analysis of data consisting of ranks of, e.g., genotypes, scored at a series of events or experiments is presented. The model postulates a latent structure, with an underlying variable realized for each genotype or individual involved in the event. The rank observed is assumed to reflect the order of the values of the unobserved variables, i.e., the classical Thurstonian model of psychometrics. Parameters driving the Bayesian hierarchical model include effects of covariates, additive genetic effects, permanent environmental deviations, and components of variance. A Markov chain Monte Carlo implementation based on the Gibbs sampler is described, and procedures for inferring the probability of yet to be observed future rankings are outlined. Part of the model is rendered nonparametric by introducing a Dirichlet process prior for the distribution of permanent environmental effects. This can lead to potential identification of clusters of such effects, which, in some competitions such as horse races, may reflect forms of undeclared preferential treatment.
L ATENT variable models for describing mechanismsby which some continuous scale maps into ordered or unordered categories of response have a long tradition in quantitative genetics and psychometry. For example, the threshold-liability model for ordinal categories dates back to Wright (1934), Dempster and Lerner (1950), Grü neberg (1952), and Falconer (1965; see Gianola (1982) for a review. Extremal models were pioneered in psychology by Thurstone (1927) and were adapted by Bock and Jones (1968) to explain choices between unordered alternatives.Suppose that an observation is an assignment into one of M mutually exclusive and exhaustive categories of response. The classical Thurstonian extremal model postulates the existence of some latent or unobserved continuous valued vector y M 31 ¼ fy i g, such that category i is observed when y i is larger than the other M À 1 elements of the vector. The probability of observing an assignment into category M, say, is given by Prðy M . y M À1 ; y M . y M À2 ; . . . ; y M . y 1 j parametersÞ;