Time differential morbidity and mortality
Time of presentation with an illness is associated with differential morbidity and mortality in most studies. Population cohorts separated in time can have quite different characteristics and social change can affect age cohort mortality substantially. Accordingly those that study this issue need to understand the reasons for changes in incidence rates or prevalence. The "weekend effect" applies to certain presentations but is easy to misunderstand if severity of presenting illness is not controlled for adequately.