PrepTest 131, Section 2, Question 20
Quality control investigator: Upon testing samples of products(55 more words not shown)
The reasoning in the quality(9 more words not shown)
- Abases its conclusion(11 more words not shown)
- Bpresumes, without providing(24 more words not shown)
- Coverlooks the possibility(15 more words not shown)
- Doverlooks the possibility(15 more words not shown)
- Epresumes, without providing(21 more words not shown)
Reading the argument
Which products did the inspectors send to the laboratory? If they picked items that looked damaged or unusual on the assembly line, the laboratory could receive a much worse batch than the supplier typically produces.
The finding is that over 20 percent of the submitted samples are defective. The contract, however, concerns the defect rate among all the items the supplier manufactures for this customer. Those percentages need not match. Inspectors who focus on suspicious items could send a sample with a high defect rate even when the supplier's overall rate stays below 5 percent.
What we’re looking for
For the flaw, I want that selection problem: the argument assumes the samples represent overall production without considering that the inspectors may have sought out likely defects.
Working the answers
- A
We aren't told how many items were tested. The sample might be large. Even a large sample could be biased if inspectors chose it because the products looked faulty, so 'too small' isn't the problem we can identify from the passage.
- B
At first this can sound like the fair-sampling assumption we wanted. But being equally likely to select a defective item as a nondefective item would give the two categories equal selection probabilities. A representative selection needn't do that. If defects are rare, an unbiased selection should usually pick a nondefective item. The argument doesn't assume a fifty-fifty split between the categories. The relevant concern is whether suspected defects are overrepresented.
- C
A few sites might produce most of the defective items, but those items still count toward the supplier's overall defect rate. If that overall rate violates the contract, concentrating the problem at a few sites wouldn't excuse it. C doesn't explain why the sampled rate might overstate the overall rate.
- DCorrect
This gives the actual selection mechanism we were looking for. If inspectors tend to choose items they suspect are defective, the laboratory samples may contain a disproportionately large share of faulty products. The over-20-percent finding then wouldn't establish that the rate among all items manufactured for this customer exceeds the contractual limit.
- E
Equal visits to every site aren't required to obtain a representative product sample. Sites might make different numbers of items, and a visit might yield different numbers of samples. The argument doesn't rely on identical visit counts. We need to know whether the chosen items fairly represent production, not merely how often inspectors appeared at each location.
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