In randomized controlled trials, subjects treated differently are followed up over time to observe the effects of the intervention (treatment). ‘Following up people's health’ points out that people can be observed over time even without any treatment being administered at the start of the study. These observational follow-up studies are called cohort studies. A group of subjects is chosen and a number of characteristics ‘exposures’ of the subject measured and recorded. The subjects are followed up over time, and a number of events recorded, typically the occurrence of a disease, like diabetes or myocardial infarction, or death from a disease, or change in some trait like weight or blood pressure.