Table measuring risk, rate and odds ratio

Cohort studies allow for 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 the measurement of risk or rates. They, however, allow the 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 control 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 Formulation	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 (Exclusive)	People disease free throughout study period

Where: Ce=Cases exposed Cu=Cases unexposed Ne=Population exposed Nu=Population unexposed PTe=Person-time exposed PTu=Person-time unexposed