I'm working on a study that is drawing from another very large study, and proposing a case-cohort design. Briefly, the main cohort consists of ~40,000 individuals, ~1250 of whom had an event over the duration of the study. The intention is to develop a survival model based upon the event time distribution of the dataset.
Regarding selection of cases, the study from which we plan to draw samples extends for ~10 years. Our interest is to focus on the near term events, specifically events occurring in 5 years or less. The question that has come up regarding cases is whether we only use the subset of cases having events within 5 years or if including additional cases beyond that time is beneficial. The target population for use of the model is certainly the under 5-year event group, but it has been argued that we should use all cases regardless of when their event occurred - even well after 5 years - in order to more accurately model the survival curve.
Does inclusion of cases outside the intended use population lead to bias in the model? Would inclusion of those samples beyond our expected time cutoff provide a benefit in a model intended to be used for near term events?