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A case series describes characteristics of a group of patients with a particular disease or patients who have undergone a particular procedure. A case series may also involve observation of larger units such as groups of hospitals or municipalities, as well as smaller units such as laboratory samples. Case series may be used to formulate a case definition of a disease or describe the experience of an individual or institution in treating a disease or performing a type of procedure. Case series should comprise consecutive patients or observations seen by the individual or institution to minimize . A case series Less

Case-control studies, which are always retrospective, compare those who have had an outcome or event () with those who have not (). Cases and controls are then evaluated for exposure to various and thus should never be selected on the basis of their exposure to the risk factors under investigation. Cases and controls generally are matched according to specific characteristics (eg, age, sex, or duration of disease) to reduce by these variables. However, if the matched variables are inextricably linked with the exposure of interest (not necessarily with the disease or outcome of interest), matching may confound the analysis (see Less

A prospective cohort study follows a group or of individuals who are initially free of the outcome of interest. Individuals in a cohort generally share some underlying characteristic, such as age, sex, or exposure to a . Some studies may comprise several different cohorts. The study is usually conducted for a predetermined period, long enough for some members of the cohort to develop the outcome of interest. Individuals who developed the outcome are compared with those who did not. The report of the study should include a description of the cohort and the length of follow-up, what were measured and Less

In an observational study, the researcher identifies a condition or outcome of interest and then measures factors that may be related to that outcome. Although observational studies cannot lead to strong causal inferences, they may nonetheless suggest certain causal hypotheses. To infer causation in observational studies, investigators attempt to establish a sequence of events if event A generally precedes event B in time, then it is possible that A may be responsible for causing B. Such studies may be either (the investigator tries to reconstruct what happened in the past) or prospective (the investigator identifies a group of individuals and Less