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#1
Stratification
Oral Contraceptive (OC) use is hypothesized to increase the risk of stroke. Assume that oral contraceptive use is inversely associated with age (younger women are more likely to use OCs compared to older women), and that age is positively associated with stroke (older women are more likely to have a stroke compared to younger women).
  • Is age associated with the exposure? The outcome? Is it in the causal pathway of interest? (1 point)
  • Calculate the crude OR (2points)
Stroke No Stroke Total

OC Use 86 683 769
No OC Use 93 959 1052
Total 179 1642 1821


  • Ages 20-39
    Ages 40-49
    Ages 50-54

    Stroke
    No stroke
    Stroke
    No stroke
    Stroke
    No stroke
    OC Use
    13
    51
    31
    301
    42
    331
    No OC Use
    46
    285
    30
    463
    17
    211
    Total
    59
    336
    61
    764
    59
    542
    Stratify by the potential confounder, calculate stratum-specific ORs Stratified by age
    Odds ratio (age 20-39)
    Odds ratio (age 40-49)
    Odds ratio (age 50-54)
    Summary (age-adjusted) odds ratio* = 1.57
    * The summary OR was calculated using a statistical procedure known as the Mantel-Haenszel weighted odds ratio. You will learn about this measure in future epidemiology and biostatistics classes. (6 points)
    • As the investigator, which measure of effect (crude or adjusted) would you choose to report to your audience? (2 points)