PrepTest 128, Section 2, Question 11
A recently completed study of several hundred subjects,(34 more words not shown)
Which one of the following,(6 more words not shown)
- AThe subjects who(15 more words not shown)
- BThe subjects who(19 more words not shown)
- CA large number(11 more words not shown)
- DWhether or not(18 more words not shown)
- EA person who(15 more words not shown)
Reading the argument
The exercisers were less likely to die. That is a difference worth investigating, but was exercise responsible for it?
Perhaps the people who exercised also ate healthier food. Or imagine that an illness happened to spread through the nonexercising group. Either difference could help explain why more people in that group died. Simply finding exercise and longer survival together doesn't isolate exercise as the cause.
The subjects' similar ages also make me wonder how broadly the result applies. But be careful with the actual conclusion: exercise 'can' increase life span. That doesn't require showing that it helps every age group. The more direct issue is whether the study supports an effect of exercise at all.
What we’re looking for
To strengthen the argument, I'd like to reduce the concern about other differences between the groups. Evidence that the groups were otherwise comparable would help, as might consistent results from further studies. Let's see what these answers provide.
Working the answers
- A
This gives us exactly the kind of alternative explanation we worried about. If the nonexercisers ate less healthy diets, their higher death rate might reflect diet rather than the absence of exercise. A makes it harder to isolate exercise as the cause.
- B
Blaming lack of time explains why subjects say they didn't exercise. It doesn't show that failing to exercise caused their higher death rate. We still need evidence connecting the exercise difference to survival, not merely a reason given for the behavior.
- C
Preexisting conditions could matter, but how were they distributed between the groups? C doesn't say. Nor does the existence of an illness rule out exercise affecting its course. Knowing that many deaths involved such conditions doesn't, by itself, strengthen the claim that exercise explains the difference in survival.
- DCorrect
Random assignment helps address the alternative-cause concern. The researchers weren't deliberately putting healthier people in the exercise group or letting the most health-conscious subjects select themselves into it. A random process makes systematic starting differences less likely, so the later survival difference provides better evidence about exercise. It doesn't guarantee perfectly identical groups or prove the conclusion, but it strengthens the study's causal interpretation.
- E
Concern for health is a reason someone might exercise, not evidence that the exercise actually extends life. A health-conscious person might also eat differently or take other precautions. The motive doesn't isolate exercise's effect and could accompany the very differences we wanted to investigate.
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