Question 38·Medium·Evaluate Statistical Claims: Observational Studies and Experiments
A nutrition researcher surveyed a simple random sample of 1,000 adults who live in City X. For each adult, the researcher recorded the person’s average daily number of servings of fruits and vegetables and the person’s total blood cholesterol level. The researcher found that, on average, adults who reported eating at least 5 servings per day had total cholesterol levels that were 14 milligrams per deciliter lower than those of adults who reported eating fewer than 5 servings per day.
Based on this study design and the data collected, which of the following conclusions is best supported?
For questions about conclusions from studies, first identify the study type: look for words like “surveyed,” “recorded,” or “observed” (observational study) versus “assigned,” “gave,” or “treatment” (experiment). If it’s an observational study with a random sample, you can generalize to the population but should only claim an association, not causation. Quickly eliminate answer choices that use strong causal or guaranteed language like “causes,” “will decrease,” or “for all,” or that contradict the reported data. Choose the option that accurately reflects a relationship in the correct population without overclaiming what the study can prove.
Hints
Think about what the researcher did and did not do
Did the researcher assign some adults to eat more fruits and vegetables and others to eat less, or did they just record what people reported and their cholesterol levels? How does that affect whether we can talk about cause and effect?
Focus on words that show cause and certainty
Look for words like “lowers,” “will decrease,” or “for all” in the answer choices. Ask yourself: does this study design justify that strong a claim, or can it only show a relationship between variables?
Use the actual finding in the prompt
The researcher found that one group had cholesterol levels 14 milligrams per deciliter lower on average than another group. Which choices clearly agree that there is some relationship between intake and cholesterol, and which either deny any relationship or overpromise what the study can prove?
Step-by-step Explanation
Identify the type of study
The researcher surveyed a simple random sample of 1,000 adults and recorded each person’s usual fruit/vegetable servings and cholesterol level.
- The researcher did not assign people to eat a certain amount.
- That means this is an observational study, not an experiment.
In an observational study, we look at what people are already doing and measure outcomes, instead of controlling what happens to them.
Know what conclusions are allowed
From a simple random sample, we can usually generalize results to the population (here, adults in City X).
From an observational study (no random assignment to treatments):
- We can say there is a relationship/association between two variables.
- We cannot conclude that one variable causes the other to change.
So any choice that claims a definite cause-and-effect is too strong for this study design.
Match the choices to what the data show
The data show that people who reported eating at least 5 servings per day had average cholesterol that was 14 milligrams per deciliter lower than those who ate fewer than 5 servings per day.
So the correct conclusion must:
- Acknowledge that there is some relationship between eating more fruits/vegetables and cholesterol levels.
- Refer to the appropriate group: adults in City X.
- Avoid claiming that eating more fruits/vegetables definitely causes cholesterol to go down or that it will happen for every individual.
Select the conclusion that fits
Only the statement “Eating at least 5 servings of fruits and vegetables per day is associated with lower cholesterol among adults in City X.” correctly describes an association (not guaranteed causation) in the population being studied and matches what the data show.