Question

Calculate the relative risk of being non-active versus being “highly active” (i.e., vigorous physical activity at least twice a week) in men and in women and how would you interpret the relative risk in men and in women [4 marks]

Relative risk of non-active and highly active= 80/ (80+59) /42/ (42+59)

                                                                           = 0.57/0.41

                                                                           = 1.39     

                                                                                                    

(f) Looking at the sex and age adjusted HR in Table 3 (the HR stands for Hazard Ratio which is similar to rate-ratio) (i) how would you describe the association between physical activity and CHD   [4 marks]

after analysing the table 3 crude rate and sex and age adjusted ratio is similar. It concluded that physical activity decreased the both crude rate and even decreased the incidence rate of coronary heart disease.

           

(g) The authors removed from the analysis any person who self-rated their health as “bad” or “anywhere between good and bad” why? (no more than 60 words) [4 marks]

(h) What possible bias could have changed the estimate for the association between physical activity and CHD – at least one, and explain why. [4 marks]

(i) Do you think there is a risk of confounding bias in this research? Any answer yes or no must be justified [4 marks]

Answer 1

(f) A small amount of physical activity also favorably affects those at high risk of CHD even in the presence of other risk factors. Physical activity helps in reducing blood pressure, lowering insulin levels, and increasing good (HDL) cholesterol. It also helps in reducing the fibrinogen and lowers the viscosity of plasma. Thus, the conclusion of decreased crude rate and incidence rate of CHD holds true here.

(g) The author considered those subjects who self-rated their health as “bad” or “anywhere between good and bad” as persons with poor self-reported health status. The high age of patients at the start of the study and long follow up implies a risk of confounding caused by an unknown disease process that's why poor self-reported health status patients were removed.

(h) Presence of other CHD risk factors like hypertension or diabetes or smoking, etc may induce a bias in correlating association of physical activity and CHD. Patients with the above-said conditions are already at a higher risk for CHD so the cardioprotective effect of physical activity in such patients might be lower when compared to patients who do not have hypertension or diabetes or smoking. Also, the amount of physical activity done by such patients will be reduced when compared to patients who do not have these conditions, which introduces a bias.

One more bias can be because of the non-response of the subjects. It depends on what type of physical activity is considered in the study. Generally, household activities or gardening are not included. These activities also require some amount of physical activity (but to what significant amount, depends on the study).

(i) Confounding bias causes distortion by the addition of a third factor while measuring the influence of one factor on health causing mixing the effect of the primary factor with other factors. Yes, there is a risk of confounding bias. Here, we are studying the effect of physical activity on CHD. For example, the presence of smoking may confound the association between physical activity and CHD. Subjects doing hard physical activity might be smokers which can distort the results because smoking is independently linked with CHD.