Question

. In a hypothetical case-control study of 180 cases of heart disease and 300 unmatched controls, the following 2x2 table was determined for the association of sedentary lifestyle with heart disease:

	Cases	Controls
Sedentary lifestyle	135	150
Not sedentary lifestyle	45	150
	180	300

(a) Compute the odds ratio for sedentary lifestyle and heart disease.

(b) There is concern that sex may be a confounder in the relationship between sedentary lifestyle and heart disease. What three conditions must be true for sex to be a confounder in this study?

When the subjects were stratified by sex, the following tables were obtained:

Females:

	Cases	Controls
Sedentary lifestyle	15	30
Not sedentary lifestyle	30	120
	45	150

Males:

	Cases	Controls
Sedentary lifestyle	120	120
Not sedentary lifestyle	15	30
	135	150

(c) Compute the odds ratio for sedentary lifestyle and heart disease for each of these tables separately.

(d) Does the relationship between sedentary lifestyle and heart disease appear to differ for females and males? (That is, is there an interaction between sex, sedentary lifestyle, and heart disease?) Why or why not?

(e) Does sex confound the relationship between sedentary lifestyle and heart disease seen in part (a) above? Why or why not?

Answer 1

Odds ratio is cross product ratio , it is a measure of strength of association between risk factor and outcome. it is a key parameter in analysis of case control studies.

a. Odds ratio = 135×150/150×45

=20250/6750 = 3

b..a confounding factor is one which is associated with both exposure and disease. Sex ,smoker , or alcholic.

C.odds ratio for females = 15×120/30×30 =1800/900 =2

Odds ratio for males= 120×30/120×15

3600/1800 =2

This means that there is association

Between sedentary life style and heart disease.

d.. risk factor for females. 15/45 =1/3

risk factor for males is 120/150 =4/3

Hence it is different for males and females., males are more prone .hence sex is a confounding factor.