Figure 1:

Comparison of methods to model the relation between a continuous confounder (use of health care) and outcome (death) in a study of influenza vaccination. Use of health care was defined as the number of contacts with general practitioners in the 12 months before the influenza vaccination. Use of health care was dichotomized at the median (i.e., 12 contacts in the 12 months before the vaccination). Use of health care was divided into 5 categories based on quintiles; cut-values were 5, 9, 14 and 22 contacts in the 12 months before the vaccination. CI = confidence interval.