The statistical analysis was run with logistic regression
models (LOGISTIC procedure, SAS, 2000) to determine the
relative contribution of each factor to the probability of a
swine female having a return to estrus after mating.
Backward elimination was used to determine which
factors could be excluded from the model based on a
likelihood ratio Chi-square statistic corresponding to
P = 5% at each step. The dependent variable was defined
as the occurrence of return to estrus at any moment of
gestation. All the potential risk factors were included in the
models as categories