Case nested and case-cohort studies are a variation of case-control study in which cases and controls are derived from an established cohort study. The main difference is in the control selection. Case cohort is more convenient, the same sub-cohort can be used to study more than an outcome, if you can’t find a risk set for each control you can recruit the sub-cohort at the time you recruit the original cohort and more flexible analysis. Nested case-control is used to reduce costs and reduced time to get results. Compared to a case-control study nested case-control has less prone to recall and observer bias and the temporality of the relationships is better confirmed.