A relative risk of less than 1.0 provides evidence for a protective effect of exposure (the incidence rate of disease among exposed is lower than non-exposed) whereas a relative risk above 1.0 suggests that exposed people are at higher risk of disease than non-exposed persons.Äepending on the time when the cohort study is initiated relative to occurrence of the disease(s) to be studied, one distinguishes between current and historical cohort studies. A relative risk of 1.0 signifies that the incidence rate is the same among exposed and non-exposed subjects and indicates a lack of association between exposure and disease. The relative risk is the ratio of the incidence rate of index subjects to that of control subjects. The measure of association between exposure and disease in cohort studies is the relative risk. The incidence rates for exposed and non-exposed subjects are calculated separately. The numerator of the rate is the number of diseased subjects and the denominator is usually the number of person-years of observation. The measure of disease in cohort studies is the incidence rate, which is the proportion of subjects who develop the disease under study within a specified time period. In order to compare the occurrence of disease in exposed subjects with its occurrence in non-exposed subjects, the health status of a group of individuals not exposed to the agent under study (control subjects) is followed in the same way as that of the group of index subjects. Individuals exposed to the agent under study (index subjects) are followed over time and their health status is observed and recorded during the course of the study. The starting point of a cohort study is the recording of healthy subjects with and without exposure to the putative agent or the characteristic being studied. For a more detailed account of design, conduct and analyses of epidemiological studies, the reader is referred to textbooks and methodological articles given in the list of references. The methodological principles of cohort and case-control studies are briefly outlined. The cohort study starts with the putative cause of disease, and observes the occurrence of disease relative to the hypothesized causal agent, while the case-control study proceeds from documented disease and investigates possible causes of the disease. The two epidemiological methodologies to study disease causation outlined in this chapter have different approaches. Although information of such extraneous factors is collected and quantitatively adjusted for when they are known to be present, findings from observational epidemiological studies are generally less conclusive than those from experimental studies because of the less strict control of extraneous Observational epidemiological research has the disadvantage that extraneous factors cannot be manipulated by the investigators. In biomedical research on human beings the randomized clinical trial is the closest option to experimental research methodology. Such extraneous factors may, if not under control, distort the results of the research and lead to false conclusions about cause and effect. The experimental approach allows control of the effect of extraneous factors that may have an effect on the outcome under study, but are not under investigation. Through repeated manipulation of one or more factors in a series of experiments concerned with subquestions, the main research question can be resolved. In experimental research, investigators can manipulate one factor while controlling others, and the main research question can be broken down into subquestions with comparatively simple causal assumptions (12). Experimental research provides data from which firmer conclusions can be made as compared with epidemiological studies. Epidemiological research is, to a large extent, of an observational character as opposed to experimental research. This is not to say that findings from cohort and case-control studies always reflect true associations which can be universally generalized. The conclusions that can be drawn from findings of these types of studies are, however, much weaker compared to those of cohort and case-control studies. Other important types of epidemiological studies mainly for generating hypotheses include cross-sectional and ecological, or correlation studies. World Health Organization, 1211 Geneva 27, SwitzerlandĬohort and case-control methodologies are the main tools for analytical epidemiological research. Special Programme of Research, Development and Research Training in Human Reproduction, Training course in research methodology and research protocol development 2021
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