Question

Researchers investigating the development of the idiopathic inflammatory myopathies (IIMs) such as polymyositis read that vitamin D may act as an immunomodulator that reduces the development and severity of autoimmune diseases. Given that many Americans are vitamin D deficient, the researchers design an observational study to assess the impact of vitamin D supplementation on IIM symptom severity. Subjects are surveyed at time 0 and after two years, and the results are listed in the chart. Which equation represents the chance of symptom improvement in subjects who took vitamin D supplements relative to subjects who did not take vitamin D supplements?

	Vitamin D supplemen taken	Vitamin D supplement not taken
Symptoms improved	50	60
Symptoms not improved	300	400

(A) (50 / 300) / (60 / 400) 

(B) (50 / 350) / (60 / 460) 

(C) (50 / 460) / (60 / 300)

(D) (60 / 400) / (50 / 300) 

(E) (60 / 460) / (50 / 350)

Answer 1

1. The correct answer is B. This prospective, observational study is a cohort study. Therefore, the likely unit of measure is the relative risk, which is the risk of a health outcome with a given exposure versus the risk of a health outcome without the exposure. In this case, the relative “risk” of a health outcome is really the relative chance of improvement of symptoms. To calculate the relative “risk,” first calculate the chance of improvement with vitamin D supplementation by dividing the number of subjects receiving supplementation whose symptoms improved (50) by the total number of subjects taking vitamin D; this is 50/350. Then calculate the chance of improvement without vitamin D supplementation by dividing the number of subjects not receiving supplementation whose symptoms improved by the total number of subjects not taking vitamin D; this is 60 / 460. The ratio of these values is the relative risk: (50 / 350) / (60 / 460). 

Answer A is incorrect. This value does not appropriately calculate the relative risk with vitamin D supplementation. The value is equivalent to the odds ratio, which is the measure typically used to analyze a retrospective, casecontrol study. 

Answer C is incorrect. This value does not appropriately calculate the relative risk with vitamin D supplementation. It does not represent any commonly used measure of analysis.

  Answer D is incorrect. This value does not appropriately calculate the relative risk with vitamin D supplementation. Instead, the value is essentially equivalent to the inverse of a calculation for odds ratio, which is not a measure used in data analysis.