Effect measure modification occurs when the strength of an association varies according to the level of a third variable which is called an effect measure modifier. Confounding is the distortion of the association between an outcome of interest and an exposure because an important third variable has not been accounted for; this third variable is called a confounder.
The association between heavy alcohol consumption and the risk of oral cancer was investigated in a case-control study with 475 cases and 400 controls. The following results were seen:
Heavy Alcohol Consumption |
Cases |
Controls |
Yes |
350 |
200 |
No |
125 |
200 |
Answer a)
We will calculate odds ratios (OR) using a two-by-two frequency table
Where
a = Number of exposed cases
b = Number of exposed non-cases
c = Number of unexposed cases
d = Number of unexposed non-cases
OR = (a/c)/(b/d) = ad/bc
Applying above formula to our case, we get
OR = (350*200)/(125*200) = 2.80
Interpretation
Heavy drinkers had 2.8 times the odds of oral cancer compared to those who were not heavy drinkers.
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