Introduction in order to study the association between disease status (eg, nosocomial infection) and some exposure variable (eg, number of days on urinary catheter), it is often necessary to take into account other variables that may influence either the disease status or the exposure variable. For example, Freeman et al described a retrospective (in their terminology “case-referent”) study of a neonatal population. In this study, cases of nosocomial infection are selected in addition to corresponding control individuals who did not experience a nosocomial infection. All patients were selected from a neonatal intensive care unit, and the goal was to study the role of umbilical artery catheterization and its association with nosocomial infection. As noted by Freeman et al, this is a complex question because the risk of nosocomial infection might reasonably depend not only on the duration of catheterization, but also on the birth weight of the infant.