Question

in countries known to contain lower levels of vitamin d, there were more reported cases of a particular type of rare disease. some researchers hypothesized that a deficiency in vitamin d would increase the risk of developing the disease. these researchers compared vitamin d levels in blood samples from 110 military personnel who have developed the disease with blood samples of nearly 310 who have not. the samples were taken, on average, five years before the disease was diagnosed. those with the highest blood vitamin d levels had a 61% lower risk of developing the disease than those with the lowest levels. complete parts a) through d)

b) is that an appropriate choice for investigating this problem? explain.
a. yes, because the disease in the study is a rare disease, a retrospective study is appropriate.
b. yes, because the disease in the study is a rare disease, a prospective study is appropriate.
c. no. for rare diseases, it is better to perform an experiment rather than a retrospective study.
d. no. for rare diseases, it is better to perform an experiment rather than a prospective study.

c) who were the subjects?
a. all people who developed the disease
b. all people representative of the countries in the study
c. the researchers
d. military personnel

d) what were the variables?
a. the level of vitamin d for the subject and whether or not the subject developed the disease
b. the number of subjects who developed the disease and the number of subjects who did not develop the disease
c. the level of vitamin d for the subject and the percent lower risk in obtaining the disease
d. the level of vitamin d for the subject, the number of subjects who developed the disease, and the number of subjects who did not develop the disease

Explanation:

Part b)

For rare diseases, retrospective studies are appropriate as they look back at existing data (like past blood samples here) to analyze associations. Prospective studies would require following subjects over time, which is hard for rare diseases due to low occurrence. Experiments (like assigning vitamin D levels) might be unethical or impractical here. So option A is correct as it states a retrospective study is appropriate for a rare disease.

The passage says samples were taken from 110 military personnel with the disease and ~310 without. So the subjects are military personnel. Other options: A is too broad (only military here), B is incorrect (not all people in countries), C is wrong (researchers aren't subjects).

Variables are what's measured/studied. Here, we look at vitamin D levels (a variable) and whether the subject developed the disease (another variable). Option B is about counts (not variables), C includes "percent lower risk" which is a result, D includes counts. So A is correct.

Answer:

A. Yes, because the disease in the study is a rare disease, a retrospective study is appropriate.

Part c)