Question

in counties 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).
a) what kind of study was this? (select all that apply)
prospective
cross - sectional
experiment
retrospective
b) is this an appropriate choice for investigating the 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

Explanation:

Part a)

• Case - Control Study: In a case - control study, we compare individuals with a disease (cases) and those without (controls) and look at their past exposures (here, vitamin D levels in blood samples taken before disease diagnosis).
• Retrospective Study: A retrospective study looks back in time. Here, blood samples were taken years before the disease was diagnosed, and the researchers are looking at past data (vitamin D levels) in relation to the current disease status.
• Not Prospective: A prospective study follows a group forward in time to see who develops the disease, which is not the case here.
• Not an Experiment: In an experiment, we manipulate a variable (e.g., give vitamin D supplements), but here we are just observing and comparing existing data.

The disease is rare. In a prospective study, we would need to follow a large number of people for a long time to get enough cases of the rare disease, which is impractical. A retrospective study is appropriate because we can look at existing data (blood samples taken before diagnosis) of people who already have the disease (cases) and those who don't (controls). So option A is correct as it states that since the disease is rare, a retrospective study is appropriate.

The problem states that the samples are from 110 military personnel who have developed the disease and nearly 310 who have not. So the subjects are military personnel. Option D is correct. Option A is wrong because it's not all people with the disease, just military personnel with the disease. Option B is wrong as it's not all people representative of the countries, it's military personnel. Option C is wrong as the researchers are not the subjects.

Answer:

Case - Control, Retrospective (the options marked in the problem for part a are "Case - Control" and "Retrospective")

Part b)