PrepTest 119, Section 3, Question 3
A recent study involved feeding a high-salt diet(69 more words not shown)
The answer to which one(13 more words not shown)
- AHow much more(15 more words not shown)
- BDid the high(13 more words not shown)
- CWhat percentage of(8 more words not shown)
- DHow many rats(13 more words not shown)
- EHave other species(10 more words not shown)
Reading the argument
What was their blood pressure before the salty diet?
We have a result after a few months: 70 percent of the rats had high blood pressure, and another 5 percent had extremely high blood pressure. That's 75 percent with abnormally high readings. But we don't yet have a comparison showing whether the diet changed anything.
Suppose only 5 percent had abnormal blood pressure beforehand. An increase to 75 percent would make the diet worth investigating as an explanation. But suppose 75 percent already had abnormal blood pressure before the experiment. The result afterward would tell a very different story. Looking only at the final numbers conceals that difference.
What we’re looking for
The question asks what would help us evaluate the conclusion linking high-salt diets with high blood pressure. I'd want the starting numbers. We don't have to identify every possible problem in the study; we need the question whose answer would most directly help us judge what these results show.
Working the answers
- A
The exact amount of extra salt could be useful for designing or describing the experiment. But we already know the diet was high in salt. Whether it was moderately or dramatically higher doesn't tell us whether the rats' blood pressure was already abnormal before the diet began. The missing blood-pressure comparison is more directly relevant to the inference.
- B
Harmful consequences of high blood pressure are separate from its relationship with salt. Even if none of these rats suffered further harm, the diet could still be linked to their blood-pressure readings. And serious health effects wouldn't by themselves show that the diet explains those readings.
- C
How often wild colonies eat high-salt diets concerns how common that diet is outside the experiment. It doesn't supply the starting blood pressure of the rats being studied. A rare diet could have an effect, and a common diet could have one too; frequency of exposure doesn't answer our question about this result.
- DCorrect
This supplies the missing baseline. If very few rats had abnormally high pressure before the study and 75 percent did afterward, that change would support investigating a connection with the diet. If roughly the same proportion had abnormal readings from the start, the reported result would give much less reason to infer a new diet-related difference.
Neither answer would settle every possible explanation on its own. But the contrast shows why knowing the initial readings matters to evaluating the evidence.
- E
Other rodent experiments might eventually provide useful evidence, but simply knowing whether they occurred gives us no results. The conclusion here concerns rats, and E doesn't tell us what happened in any other species or clarify the missing comparison in this rat study.
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