Univariable linear or logistic regression with ELISA PP as the
explanatory variable was performed separately for each production
and fertility outcome variable. Cases with missing data were
excluded. Yield was added into each model as an additional
explanatory variable, to control for confounding, as there is known
to be a relationship between yield and most production and fertility
variables (Nebel and McGilliard, 1993). Risk factors for fluke
infection were also added to the models because some of these, for
example herd size and grazing management, could also have an
effect on yield.