PrepTest 104, Section 1, Question 3
In Yasukawa's month-long study of blackbirds, the percentage(48 more words not shown)
The statements above, if true,(7 more words not shown)
- AAmong the blackbirds(13 more words not shown)
- BLarger blackbirds of(21 more words not shown)
- CAmong blackbirds of(21 more words not shown)
- DAmong blackbirds of(20 more words not shown)
- EWith a larger(25 more words not shown)
Reading the argument
Why did more of the smaller birds survive the month? Perhaps their size helped them hide from predators. Or perhaps the smaller birds were younger, and age—not size—accounted for the survival difference. Those are different explanations of the same observation.
Yasukawa concludes that size helps determine a blackbird's chances of surviving. The critic points out that smaller blackbirds are generally younger. That gives us a reason to pause before attributing the result to size: the comparison may also be comparing ages.
What we’re looking for
We're asked what inference the statements support. My initial thought is that age may explain why the smaller birds did better. We should carry that possibility into the answers without turning it into a proven claim that age is the only factor that matters.
Working the answers
- A
We aren't told that the relationship between size and age disappeared among the survivors. Smaller surviving birds could still generally be younger than larger surviving birds. Separating out those that lived through the month doesn't establish the absence of a relationship within that group.
- B
Nothing establishes that larger birds of the same age are more likely to survive. That is a specific advantage for larger size, and we have no comparison holding age fixed that would show it. We shouldn't call it impossible; it simply isn't the inference the given evidence supports.
- C
This dismisses age differences when size is held fixed. But the criticism points to age as a possible explanation of the survival difference. We don't have evidence that age becomes unimportant among equally sized birds. C sets aside the very factor the critic raises.
- DCorrect
If the apparent advantage of smaller size comes from smaller birds generally being younger, then comparing birds of the same age may remove that apparent advantage. A size difference might no longer indicate a survival difference.
Notice “may not.” We aren't proving that all same-age birds have identical survival chances, or even that size never matters. D captures the uncertainty created by the age explanation. The study's size comparison alone may not tell us what happens when age is held the same.
- E
A larger sample doesn't automatically separate age from size. If smaller birds still generally are younger, adding more birds could preserve the same relationship and the same survival difference. The criticism isn't that too few birds were studied, and nothing establishes that a bigger study would produce identical survival percentages.
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