Non-informative priors were used for the higher-order parameters: diffuse Normal distributions for the μ b is and diffuse gamma distributions for the σ b is. Parameters in the model were estimated using Markov chain-Monte Carlo methods implemented in WinBUGS [13] . Two chains of 50,000 iterations were run for each model, with the first 10,000 iterations discarded to allow for burn-in of the chain. Each chain was then thinned by sampling every tenth iteration to reduce autocorrelation amongst the samples.