Participants: Cases (n = 448) were recruited from women aged between 16 and 44 w
ID: 448099 • Letter: P
Question
Participants: Cases (n = 448) were recruited from women aged between 16 and 44 who had suffered an incident myocardial infarction between 1 October 1993 and 16 October 1995. Controls (n = 1728) were women without a diagnosis of myocardial infarction matched for age and general practice. The study measures several variables in order to get a more precise picture. To simplify issues for this homework, let us focus on the fact that 62 of the cases and 261 of the controls used oral contraception. Calculate the appropriate measures of association, and explain why other measures may not be appropriate
Explanation / Answer
It's a Case-Control study, Population at risk is not known. Participants are selected by disease status. So we calculate Odds instead of Risk. Odds Ratio is an estimate of Relative Ratio.
Odds Ratio (OR) = Odds of disease in Exposed / Odds of disease in unexposed , or it can also be interpreted as
OR = product of two cells that support the hypothesis of an association / product of two cells that negate the hypothesis of an association
= 62 x 1467 / (261 x 386) = 0.9028.
As, in this case OR < 1 , so exposure (to Oral Contraception) is associated with decreased risk of outcome (Myocardial Infarction)
Cases (disease) Controls (non-disease) Total Oral Contraception 448 1728 1280 Exposed 62 261 199 Unexposed 386 1467 1081