Epidemiology: 13) Case-Control Studies: Design
Terms
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- 2 reasons case-conrtol trials are susceptible to bias
- 1) Extra step to select controls. 2) Both E and D are known and can influence both selection of subjects information accuracy
- 3 conditions necessary for valid case-control study
- 1) Identification of the study base. 2) Controls are random sample of study base. 3) Accurate nondifferential info on exposure and confounders.
- Identification of the study base
- 1) All cases. 2) Random sample of cases if large number
- 6 steps in a case-control study
- 1) Identify Study Base. 2)Identify cases. 3) Identify controls. 4) Exposure. 5) Confounders. 6)Analysis
- 3 keys for Disease: Cases
- 1)Aspire for high Sens and Spec. 2) Restrict to newly developed disease (or dealing with survival). 3)Try and get ALL CASES (avoids selection bias)
- Controls
- random samples from study base
- Exposure: issues for case-control
- subjects enter study after disease occurrence. Pre-disease data needed.
- Analysis
- OR and RR estimates
- Study Base
- Underlying cohort and time-period where all cases are identified and controls are randomly samples. AKA - source cohort, reference population
- What IS NOT they study base
- Cases + Controls
- 2 types of study base
- 1) Primary. 2) Secondary
- Primary Study Base
- Explicit cohort where all cases and controls can be identified. Only info needed for idenficiation is: person and dates
- Nested case-control designs
- Random sample of controls from a primary study base. Could have been a cohort study at greater cost.
- EG. of primary study base
- 1) Study conducted within an existing cohort or rct population. 2) Study conducted with medical care DB where a retro-cohort could be done
- EG. of NOT primary study base
- 1) Population where cases are ascertained is ambiguously defined. 2) Well dfined but rosters do not define study base
- Seconday Study base
- Study base identified conceptually but no definitive roster. Selcting controls approximate random sampling.
- Danger of using a secondary study base
- Sampling is not random and introduces bias
- 5 reasons to use a case-control study
- 1) When cohort study not feasible (expense, time). 2)Brief transient exposures. 3) Outbreaks. 4)Primary study base is present. 5) Seconday study base approximates random sampling.