Impact of fitting age in general linear models testing the strength of association between genotype and reactor status for the SICCT test of exposure to bovine tuberculosis.

Each model is fitted with response variable reactor status (0 = positive test, 1 = negative test) and group (either breed, ‘B’, or STRUCTURE group, ‘S’, fitted as a factor) and genotype (‘G’, fitted as a factor) either with or without age at slaughter (‘A’, fitted as a covariate. Models either included (*) or did not include (+) all possible second order interactions between terms. Significance was assessed by repeatedly resampling genotype within breed/group without replacement and refitting the model. As in Table 2, significant P-values (P