Understanding real research 2.

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Understanding real research 2.

• Case control studies.

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What can studies do?
Describe the situation Descriptive. Explain the
situation Analytical. Compare approaches
Experimental.
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Study designs.
Descriptive Cross-sectional, longitudinal. Analyti
c Case-control studies. Cohort studies. Quasi-expe
rimental Natural experiments, policy
interventions. Experimental Randomised controlled
trial.
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Study designs
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Prevalence Cross-sectional
Cause/ Aetiology Cross-sectional Case-control Cohort.
Prognosis Cohort.
Harm Case-control Cohort.
Effectiveness Randomised controlled trial.
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Past
Present
Future
Cross-sectional
Case-control
Cohort
From Altman. Practical Statistics for Medical
Research, 1991.
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Case control studies Key features.
Retrospective, i.e. backward looking. Relate an
effect or outcome to a probable
cause. Individuals with disease (cases) compared
to individuals without disease (controls).
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Case control study.
From http//library.downstate.edu/EBM2/2500.htm
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Advantages disadvantages
Advantages Disadvantages
Quicker than cohort studies No measure of incidence
Relatively small study population required Can only investigate one disease at a time no temporal association
Good for rare diseases can select all known cases Retrospective, so data may be biased, e.g. poor records memory recall
Case selection bias must define cases very explicitly
Control selection bias
Matching controls to cases can be difficult
Can only determine the odds ratio, not relative risk
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Appraising case control studies
Methodological approach Are cases and controls
similar, except for exposure to the putative
cause? Is collection of retrospective data
objective? Is there evidence of causation?
Statistical reporting Type of data influences
statistical analysis. Reporting of risk.