Question 49·Easy·Evaluate Statistical Claims: Observational Studies and Experiments
A high school launched an optional after-school math tutoring program and wants to know whether the program causes higher test scores. At the end of the semester, the school compared the average math scores of students who chose to attend tutoring with the average scores of students who did not attend. The students who attended tutoring had a higher average score.
Which statement best evaluates the school's conclusion that the tutoring program caused the higher scores?
For questions about whether a study justifies a causal conclusion, first decide if the study is a randomized experiment or an observational study. Look for words like "optional," "chose," or "volunteered"—these usually mean no random assignment, so you generally cannot claim cause and effect. Then scan the answer choices for ones that mention random assignment, confounding variables (like motivation or prior achievement), or the difference between correlation and causation, and eliminate choices that rely only on large samples, same school, or year-to-year comparisons without fixing the core design issue.
Hints
Clarify the type of claim
Ask yourself: Is the school just saying that tutoring students had higher scores, or are they saying tutoring caused the higher scores? That difference matters.
Look at how students got into each group
Focus on the word "optional." Did students get randomly assigned to tutoring and non-tutoring, or did they choose? How does that affect how confident we can be about what caused the higher scores?
Consider possible other explanations
Besides tutoring, what other characteristics might differ between students who chose tutoring and those who did not, and how could those characteristics affect test scores?
Step-by-step Explanation
Identify what the school is trying to conclude
The school is not just asking whether tutoring students scored higher; they are claiming that the tutoring caused the higher scores. This is a causal conclusion, which is stronger than saying the scores are simply associated with tutoring.
Determine what kind of study this is
The tutoring program was optional, and the school compared students who chose to attend with those who did not. That means:
- There was no random assignment of students to “tutoring” and “no tutoring” groups.
- Students self-selected into groups based on their own choices.
This is an observational study, not a randomized experiment.
Think about other possible differences between the groups
Because students were not randomly assigned, the two groups might differ in important ways besides tutoring, such as:
- Motivation or effort
- Prior math achievement
- How much they like math
- How much support they get at home
Any of these factors could influence scores. So even if tutoring students have higher average scores, we cannot be sure that tutoring is the cause.
Match this reasoning to the best answer choice
We need the choice that says the causal conclusion is not justified and explains that the key problem is lack of random assignment, which allows other differences between the groups to explain the results.
Choice C states exactly this: it points out that students were not randomly assigned and that students who chose tutoring may differ in motivation or prior achievement, which also affect scores.
Correct answer: C) The conclusion is not justified because students were not randomly assigned; students who chose tutoring may differ in ways (such as motivation or prior achievement) that also affect scores.