Cohort study

A cohort study is an observational study design in epidemiology with the goal to uncover a relationship between one or more exposures and the occurrence of a disease. One group of exposed and non- exposed persons, a group over a certain period regarding the incidence or mortality of certain diseases is observed. It is a special form of the panel study, in which all individuals of a sample belonging to the same cohort. Among a cohort is a group of persons in whose CVs a specific biographical event has occurred almost at the same time. Depending on definierendem feature distinguishes one birth cohorts, enrollment cohorts, divorce cohorts and many others.

To establish a connection, the number of new cases (incidence ) are measured in exposed and non-exposed and compared at the end of the investigation. If there is a positive association between exposure and a disease, the proportion of diseased people is the group of persons exposed to the non -exposed study group. Thus, the exposure pose a possible risk factor. Finally, to make statements about the strength of the association, the two incidence rates are taken into consideration and calculated the relative risk.

Application of cohort studies

Cohort studies are often used to study developmental psychology and generation sociological issues. A characteristic cohort study, for example, the "British National Child Development Study " ( NCDS ) of ferric and Sherpherd of 1992, which the development of 11,400 until March 9, born on 3 UK children with regard to education, certain settings, income and health examined over a period of 34 years with five time points.

In the following hypothetical cohort study, the effect of cigarette smoking (exposure) was examined for the occurrence of coronary heart disease to illustrate the procedure of the study design. The participants of the study groups showed no advance on heart disease. At the second time of investigation was asked about the incidence of coronary heart disease and the results are sorted according to the four-field table.

Using the relative risk statements about the strength of the relationship can ultimately be made. This is the ratio of the two rates: ( 28,0 / 1000): ( 17.4 / 1000) = 1.6 Thus, smokers of cigarettes have a 1.6 times higher risk of developing coronary heart disease.

Forms of cohort studies

There are two types of cohort studies: intra-and inter - cohort comparisons. In the intra- cohort comparison, the temporal evolution of certain characteristics of a cohort is examined. Inter- cohort comparisons compare the other hand, members of different cohorts together. However, a prerequisite is that the people of every defining characteristic time of the study have the same time interval.

Prospective and retrospective cohort studies

The cohort design can be carried out in two different ways. On one hand, the cohort can be assembled in the present and be accompanied to the future ( prospective cohort study). On the other hand, is being introduced in a retrospective or historical study arrangement on data from the past to evaluate these in the present. The prospective study arrangement may be partly a long observation period is based. For the first cohort is determined without knowing whether and when a disease occurs at all. A well-known example of this approach is the Framingham Heart Study, which began in 1948. The timing of data acquisition is the only difference between the two types.

Recruitment of the study population

The recruitment of the study population can also be done in two ways. The study group can be based on exposure and non - exposure, for example, smoking and Non smoking. Another possibility consists in a population, such as a city, to select, before they have been characterized in terms of various characteristics. In this case, it is possible to examine the study population in terms of several features, as also in the Framingham Heart Study, the case.

Observation over time - follow-up

The observation period is from the outcome, ie the disease -dependent. While studies on exposures during pregnancy and child-like malformation statements can be made after nine months, chronic diseases occur until much later on. Thus, also the investigation period is different. In a cohort study is also to ensure that the study population can be examined or interviewed at least at two points in time. Because the only way to statements about a possible connection can take.

Distortion options in cohort studies

There are different distortions (English bias) that may arise in this study design. For example, can be a distortion due to different quality of the information at the exposed and non-exposed subjects. It may also be given in the follow-up a further bias due to lack of participation or early withdrawal. This complicates the interpretation of the study results significantly. The distortion must either be avoided or accepted.

Interpretation and problems

The changes that are observed in cohort studies, are due to three factors:

• Effect of age or life-cycle effect (eg, increased anxiety, with 18 years include many from a building loan contract, risk of divorce in the 3rd year of marriage )
• Cohort effect or vintage effect (eg '68 setting, The Year 1989 (1999 ten years of age ), above average number of movie tickets for the movie "1989" )
• Period effect or year effect: (eg hoarding against disasters; In 2000 bought over -average number of a 2000 calendar)

The term " age effects " refers to the effects of aging. Period effects are assumed when the changes with the calendar time correlate. Cohort effects refer to the effects of cohort membership. A problem in the interpretation of cohort studies is that, the three effects mentioned separate into individual studies only under additional assumptions. This is referred to as the identification problem.

Advantages and disadvantages of cohort studies

A cohort study allows a direct determination of the rate of new cases (incidence) and thus provides a way to determine the absolute risk of exposure to diseases. However, in order to obtain these results, many study participants are often necessary, so that the study design is expensive and complex. Furthermore, it is disadvantageous that the results are available and suitable for non- rare outcomes only after a long time.