Many epidemiologists consider that the studies they are conducting are measurement exercises. Simple studies include measurements of disease frequency which may be expressed as risks, rates, prevalence or odds. More advanced studies will aim at identifying the causes of diseases and the effect of specific exposures on disease occurrence. This achieved by comparing disease frequency between sub groups of a population. This comparison can be expressed as a difference or a ratio, so called "effect measure".

A core function of epidemiologists is to measure the causal effect of an exposure on the occurrence of a disease. To measure a causal effect we would have ideally to compare occurrence of disease in exposed persons to what would have happened in the same persons, at the same time, in the absence of exposure. This is however theoretical since such two measurements, in the same group of persons under study, are not feasible during the same time period.In order to approach this theoretical situation as closely as possible, we will use as unexposed group a population similar to the exposed group but for the exposure. In these two populations (or in 2 subsets of the same population, exposed and unexposed), we will then measure and compare disease occurrence.

To compare disease occurrence between exposed and unexposed populations epidemiologists will have either to assign exposure or to observe populations naturally exposed. Assignment of exposure is only ethically feasible when exposure is potentially protective (treatment, vaccine, preventive measures). Observation of accidental or naturally assigned exposures will allow us to study the effect of potentially harmful exposures. To measure the effect of exposure several types of epidemiological studies are available. In cohort studies the frequency of disease is compared between a group of exposed and unexposed cohorts, while in case control studies the exposure status is compared between persons with and without disease. There are several advantages and disadvantages of cohort and case control studies which will be presented in this chapter. Furthermore, case cross over studies will be discussed in this chapter in which exposure information is obtained from the same case group but during different periods of time. The  case to case study design, a type of case control study when the disease of interest can be subclassified in two or several groups that have specific risk factors, will be highlighted briefly.