Question

in countries where vitamin d cannot be obtained from sunlight, 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 had developed the disease with 300 samples of healthy military personnel. the samples were taken, on average, five years before the disease was diagnosed. those with the highest blood vitamin d levels had a 47% lower risk of developing the disease than those with the lowest levels. complete parts (a) through (c).
(a) what kind of study was this? select all that apply.
prospective
observational
experiment
retrospective
(b) is this 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, its better to perform an experiment rather than a retrospective study.
d. no, for rare diseases, its better to perform an experiment rather than a prospective study.

Explanation:

Part (a)

To determine the study type, we analyze each option:

• Prospective: Follows a group forward in time to observe outcomes. This study compares vitamin D levels in those who developed the disease (cases) and those who didn't (controls) after the disease was diagnosed, so not prospective.
• Observational: Involves observing and measuring variables without intervening. The researchers compared existing vitamin D levels (no manipulation), so this fits.
• Experiment: Requires manipulating a variable (e.g., giving vitamin D). Here, vitamin D levels were measured, not manipulated, so not an experiment.
• Retrospective: Looks back in time to compare groups (cases and controls) on a past exposure (vitamin D levels). The study compares vitamin D levels from blood samples taken before diagnosis (looking back) between those who got the disease (cases) and those who didn't (controls), so this fits.

We analyze each option:

• Option A: A retrospective study is appropriate for rare diseases because it uses existing data (blood samples, case/control groups) instead of waiting for new cases (which would be rare and take long in a prospective study). This matches the logic for rare diseases.
• Option B: Prospective studies follow people forward, which is inefficient for rare diseases (few new cases over time). So this is incorrect.
• Option C: Experiments on rare diseases are unethical or impractical (hard to find enough subjects to manipulate). Retrospective is better, so this is incorrect.
• Option D: Prospective is not better for rare diseases (as explained for B), and experiments are not better than retrospective here. So this is incorrect.

Answer:

Observational, Retrospective (the selected options are "Observational" and "Retrospective" as per the checkmarks in the image)

Part (b)