Sex and neuter status were analyzed as categorical
data. Continuous variables of age and weight were converted
to categorical variables because nonlinear trends
were detected in the model-fitting process.Dogs were
grouped on the basis of age at date of vaccination as
follows: 2–9 months, 9 months to 1.5 years, 1.5–2.5 years,
2.5–3.5 years, 3.5–5.5 years, 5.5–8.5 years, and >8.5 years.