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21 Cards in this Set
- Front
- Back
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Other names for Case-Control Studies
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Case-comparison studies
Case-compeer studies Case-reference studies Retrospective-studies Retrospective case-control studies |
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Case control studies are _______ affected by bias
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Case control studies are HEAVILY affected by bias (more than any other study)
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ODDS RATIO (OR)
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ad/bc An estimate of the relative risk obtained by calculating the ratio of the odds of exposure among cases compared to controls
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Odds ratios values (OR values)
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Values equal to 1: no association between exposure and disease. Values <1: protective factor associated with exposure. Values >1: harmful effect
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Controls are needed in case-control studies because
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They provide a comparable estimate of the frequency of exposure in the absence of disease
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In 1976, Miettinen demonstrated that the rare disease assumption is not necessary for the odds ratio to provide a valid estimate of the relative risk.
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ODDS RATIO provides a valid estimate of the relative risk under conditions that prevail in MOST case-control studies
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Attributable risk percent Other names
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Attributable risk percent. attributable proportion. etiologic fraction.
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Attributable risk percent
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the percentage of disease or death IN THE POPULATION that is attributable to an exposure
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Unlike the AR%, the ____ is affected by the prevalence of exposure in the targeted population.
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PAR% Population attributable risk percent.
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Selection of cases
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Usually obtained from those seeking medical care. Newly diagnosed cases are preferred: if subject has had the disease for a long time, may be hard to distinguish exposures that occurred after/before the disease. Prevalent cases could lead to an overrepresentation of cases with long durations (i.e. survival bias)
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Diagnostic criteria for selection of cases
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usually obtained through medical records - may also be obtained via an investigator.
Most desirable way of obtaining cases is to include all incident cases in a defined population over a specified period of time. Most likely way of obtaining cases is to select incident cases from a hospital over a period of time. |
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Selection of controls
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Most difficult/controversial aspect of study design.
No control group for all situations. "EXPOSURE RATE THAT WOULD BE EXPECTED TO OCCUR IN THE CASES IF THERE WAS NO ASSOCIATION BETWEEN THE STUDY DISEASE AND EXPOSURE" |
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Most commonly used groups
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Probability samples of the population from which the cases come.
Persons seeking medical care at the same institution as the cases for cond. believed to be unrelated to the cases' diagnosis (most freq used group) |
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Special groups
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neighbors of cases. friends, schoolmates, coworkers.
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Approaches to sampling controls in hospital-based studies
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Select controls from a variety of different diagnostic groups so that no risk factor is overrepresented. Controls with acute conditions - so earlier exposures could not have been influenced by the condition. DO NOT select patients who have multiple concurrent conditions. Do not select patients who have a related RISK factor with the factor of interest.
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Advantages of HOSPITALIZED CONTROLS
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- readily available - more likely than healthy to be aware of prior exposure events (reduce recall bias) - have same factors as cases using that hospital - likely to cooperate.
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Disadvantages of HOSPITALIZED CONTROLS
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Likely to share similar exposures with cases (biased estimate of effect)
Decreased generalizability of study results (used hospital patients - are they really like the general population?) |
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Advantages of using the GENERAL POPULATION
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Less likely to share similar exposures wit chases - increase generalizability of results
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Disadvantages of using the GENERAL POPULATION
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More costly and time consuming - population lists from which to choose from are not always available - may not recall info as well as the cases - less motivated to participate
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Matching in the design of a case-control study does not by itself control confounding.
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Control of confounding results from matching in the design coupled with the APPROPRIATE analysis
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Matching of controls to cases (ADVANTAGES)
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Intuitively understood - Control for confounds - can provide for more adequate control of confounding by continuous variables than can adjustment in the analysis - increase accuracy of estimating the odds ratio
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