Case-Control Studies. Feature of Case-control Studies. Directionality Outcome to exposure 2. Timing Retrospective for exposure, but case-ascertainment can be either retrospective or concurrent 3. Sampling
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Outcome to exposure
Retrospective for exposure, but case-ascertainment can be either retrospective or concurrent
Almost always on outcome, with matching of controls to cases
Ideally, cases are a random sample of all cases of interest in the source population (e.g. from vital data, registry data). More commonly they are a selection of available cases from a medical care facility. (e.g. from hospitals, clinics)
Selection may be from incidence or prevalence case:
Usually, cases in a case-control study are not a random sample of all cases in the population. And if so, the controls must be selected in the same way (and with the same biases) as the cases.
Imagining the study base is a useful exercise before deciding on control selection.
The study base is composed of a population at risk of exposure over a period of risk of exposure.
For example, if cases are selected exclusively from hospitalized patients, controls must also be selected from hospitalized patients.
Matching variables (e.g. age), and matching criteria (e.g. within the same 5 year age group) must be set up in advance.
2. Controls can be individually matched (most common) or Frequency matched.
Individual matching: search for one (or more) controls who have the required matching criteria, paired (triplet) matching is when there is one (two) control (s) individually matched to each cases.
Frequency matching: select a population of controls such that the overall characteristics of the case, e.g. if 15% cases are under age 20, 15% of the controls are also
3. Avoid over-matching, match only on factors KNOWN to be cause of the disease.
4. Obtain POWER by matching MORE THAN ONE CONTROL per case. In general, N of controls should be ≤ 4, because there is no further gain of power above that.
5. Obtain Generalizability by matching by matching more than one type of control.
1. Only realistic study design for uncovering etiology in rare diseases
2. Important in understanding new diseases
3. Commonly used in outbreaks investigation
4. Useful if inducing period is long
5. Relatively inexpensive
1. Susceptible to bias if not carefully designed
2. Especially susceptible to exposure misclassification
3. Especially susceptible to recall bias
4. Restricted to single outcome
5. Incidence rates not usually calculate
6. Cannot assess effects of matching variables