It begin as cohort study, classify the subjects as exposed or non-exposed; in the follow up, some participants develop the outcome, and they are the cases and a sample of subjects without outcome are controls and with them analyzed the cases-controls study. An important advantage is that cases and controls are members of the same population, because the probability of selection bias is minimized.

Another advantage is that the information about the exposure is collected before of the develop of outcome, and the probability of information bias is minimized.