One of the most important aspects of good research is proper study design; no type or amount of statistical analysis can make up for a poorly designed study. In longitudinal research one of the most important aspects of study design is making sure that the temporal sequence of events is understood and recorded correctly. Errors in this arena can lead to misclassification of exposure time and differential follow-up between comparison groups. It is therefore crucial to understand time at risk in studies and correctly and fairly apply rules of follow-up to study subjects.