Cohort studies allow to directly measuring risk or rate of disease occurrence and their related ratio in subgroups of a population (exposed and unexposed). Case control studies do not allow measurement of risk or rates. They however allow estimation of the risk ratio and the rate ratio. The selection of the control group is a crucial step of the study. The following table summarises the type of measures and controls selection as described in the above chapter.


Measuring risk, rate and odds ratios in a case control study, using various sampling methods for controls (Source: Rodrigues L et al. Int J Epidemiol. 1990;19:205-13)


Measure Definition

Case Control


Design Controls sampled from
1-Risk ratio

Case cohort Total study population present at beginning of study

2-Rate ratio

(incidence density ratio)

Density case control People at risk at time of case disease onset
3-Odds ratio

Traditional case control People disease free throughout study period