PrepTest 109, Section 1, Question 17
Expert witness: Ten times, and in controlled circumstances,(61 more words not shown)
Which one of the following,(14 more words not shown)
- AIf similar results(17 more words not shown)
- BExpert witnesses have(13 more words not shown)
- CIn an eleventh(19 more words not shown)
- DAnother person's blood(16 more words not shown)
- ENot all expert(12 more words not shown)
Reading the argument
Ten drops behaved that way. Would another drop have to behave the same way?
The results are quite consistent: every tested drop stained between 4.5 and 4.8 square centimeters. I can see why the witness takes that as evidence. But the conclusion reaches beyond those ten observations to what a single drop of this defendant's blood will do. I'd wonder whether a different circumstance, perhaps a different kind of cut, could produce a drop that leaves a larger stain. That's a possibility to investigate, not something the passage has established.
We should also keep the claim narrow. The witness is talking about a single drop, so objecting that a spray or several drops could make a larger stain wouldn't address the conclusion. And we don't need to invent a claim about guilt; the stated conclusion is only that a drop stains much less than 9.5 square centimeters.
What we’re looking for
To weaken that conclusion, I'd look for a reason the ten small stains aren't representative. Evidence that this defendant's blood can produce a much larger stain would directly challenge the generalization.
Working the answers
- A
More matching test results would make the evidence stronger. That doesn't show that the existing results are misleading or that the conclusion is wrong. We need a reason to doubt this inference, not merely a description of how the evidence could have been better.
- B
Some experts have fudged data. Has this expert? B doesn't connect the general possibility of misconduct to these tests. It gives us no particular reason to distrust the reported results or think they fail to support this conclusion. A fact about some members of a profession is not a finding about this witness.
- CCorrect
The eleventh drop stains 9.3 square centimeters. That is still less than 9.5, but it is not much less. The conclusion includes that important qualification.
The first ten stains were around half the expected 9.5-square-centimeter area. This additional result is only 0.2 square centimeters below it. Suddenly the original ten don't look like a reliable limit on the size of stain this defendant's blood can produce. We have a direct reason to question the conclusion.
Nothing here proves the witness deliberately hid evidence. We don't need that accusation. The larger result itself is the problem for the inference.
- D
The conclusion concerns this defendant's blood. A different person's blood producing larger stains doesn't show that the defendant's blood can do so. The result could reflect a difference between people. D therefore doesn't supply the counterexample we need.
- E
This has the same problem as B: it concerns some expert witnesses, without showing that this witness lacks expertise. The question asks about the value of the evidence for this conclusion. A broad warning about credentials doesn't identify what went wrong with these observations or this generalization.
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