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Lecture 5

HMB342H1 Lecture Notes - Lecture 5: Stroke

Human Biology
Course Code
Christine Burton

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HMB342 Lecture 5 Feb 8/11 Risk: Looking Backward (Ch. 6)
Revisiting Cohort Studies
Good, direct method for estimating risk, but drawbacks:
1) Often few people develop the outcome but all must be followed.
2) Cohort studies are resource-intensive: time, effort, and money.
3) More resource-intensive (inefficient) for uncommon outcomes.
More efficient cohorts: Retrospective cohort & case-cohort, or the case-control method.
Recall: Anatomy of a Cohort Study
Cohort is assembled (none have outcome of interest, but all are at risk for the outcome).
Risk factor is identified & cohort segregated: 1) With the risk factor; 2) without the risk factor.
All members of the cohort are then observed over time to see which experience the outcome.
Rates of outcome are then estimated for the exposed vs unexposed members of the cohort.
Anatomy of a Case-Control Study (retrospective studies)
In the present, cases (all have outcome) and controls are identified (none have outcome).
Control = individual as similar as possible to each case but have not developed the outcome.
Look back in time to measure frequency of exposure to risk factor in each of case and controls.
Rates of exposure to the outcome are then estimated for the case and control & compared to estimate the
risk (but not the RR) of developing the outcome from the exposure.

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Advantages of Case-Control Studies
Efficient: instead of many people who dont get the disease (as with cohort studies), data is collected on
those with the outcome & compared to matched controls.
Speed in generating results: no wait for the outcome to develop--it has already appeared.
Disadvantages of Case-Control Studies
Bias: managing study bias is more challenging than with cohort studies.
Estimate RR, but not a true RR: case-control studies produce only an estimate of RR (odds ratio) and no
information on other measures of effect (absolute risk, attributable risk, population risk).
Designing Case-Control Studies
The validity of case-control studies hinges upon 3 aspects of the study design:
1) Selecting cases 2) Selecting controls 3) Measuring the exposure of interest.
Case-Control Study Design--Selecting Cases
Explicit criteria to define case/control, else lead to miscategorization, erroneous/biased estimate
Cases should represent all possible cases. This ensures that cases selected do not represent
extreme/atypical cases of outcome or exposure & is simpler to find matched controls.
Cases should be new (incident), not prevalent ones. Prevalent cases are more likely to contribute bias due
to factors related to disease duration.
Sample: Li et al, 2009 “BMI and risk, age of onset, and survival in pancreatic cancer.
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