6 )A study was designed to examine the association between personality type and

Question

6 )A study was designed to examine the association between personality type and coronary heart disease (CHD) using a cohort study design. In addition to a variety of other personality traits, participants were classified as competitive or noncompetitive, using a self-administered questionnaire. The following data were obtained:

Personality CHD No CHD Total

Competitive 305 345 650

Noncompetitive 58 292 350

a) Determine the incidence of CHD for each of the exposure groups. Calculate the risk ratio and interpret.

The investigators were concerned that gender might be influencing the observed association, so the data were stratified by the variable:

Males Females

No CHD No CHD

CHD Total CHD Total

Competitive 300 300 600 5 45 50

Noncompetitive    50 150 200 8 142 150

Total 350 450 800 13 187 200

b) Compute the RR for each table.

c) Is gender a confounder, effect modifier, or neither in this study?

d) How would you describe the association between competitiveness and risk of CHD, based on the findings of this study?

Epidemiology questions

Explanation / Answer

(a)

Overall incidence rate = Total CHD in each exposure group / Total observations

= (305 + 58) / (650 + 350) = 0.363

Incidence rate in Competitive exposure group = Total CHD in competitive exposure group / Total competitive observations

= 305 / 650 = 0.469

Incidence rate in NonCompetitive exposure group = Total CHD in noncompetitive exposure group / Total noncompetitive observations

= 58/350 = 0.166

b)

RR for Males

P0= probability of CHD for noncompetitive persons = 50/200 = 0.25

P1= probability of CHD for competitive persons = 300 / 600 = 0.5

Relative risk, risk ratio: RR = P1 / P0 = 0.5 / 0.25 = 2

RR for Females

P0= probability of CHD for noncompetitive persons = 8/150 = 0.053

P1= probability of CHD for competitive persons = 5 / 50 = 0.1

Relative risk, risk ratio: RR = P1 / P0 = 0.1 / 0.053 = 1.887

c)

Overall Risk ratio ,

P0= probability of CHD for noncompetitive persons = 58/350 = 0.166

P1= probability of CHD for competitive persons = 305 / 650 = 0.469

Relative risk, risk ratio: RR = P1 / P0 = 0.469 / 0.166 = 2.82

The risk ratio for the total, combined sample is RR = 2.82; this is sometimes referred to as the "crude" measure of association, because it is not adjusted for potential confounding factors. The risk ratios for the gender-stratified analysis are similar (RR = 2 and 1.887, respectively), but less than the crude risk ratio. This indicates that there was confounding by gender in the overall sample. As, the risk ratios for the gender-stratified analysis are similar (RR = 2 and 1.887, respectively), there is no effect modifier.

d)

As, RR is greater than 1, this means that competitive persons have a higher risk than non-competitive persons. Also, RR is near 2, means that the risk of CHD is doubled. In other words, the risk of CHD is increased by 100% or increased to 200%.

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