PrepTest 130, Section 3, Question 5
Motor oil serves to lubricate engines and thus(55 more words not shown)
Which one of the following,(6 more words not shown)
- ACheaper brands of(13 more words not shown)
- BTests other than(16 more words not shown)
- CThe lubricating properties(16 more words not shown)
- DThe engines of(25 more words not shown)
- EAbility to retard(17 more words not shown)
Reading the argument
If two oils protect an engine equally well, I can see the appeal of buying the cheaper one. But let's look at what the study actually establishes before deciding which is the best buy.
The oils were tested in taxicabs over 6,000 miles. I'd want comparable driving conditions: if one taxi were driven much faster or under harsher conditions, that could affect engine wear independently of the oil. The passage doesn't tell us about those details, so we shouldn't claim to have found an actual defect in the study. It is a question worth asking.
Even accepting the reported result, I have two further concerns. What happens over longer distances? Equal performance over 6,000 miles doesn't automatically establish equal performance over every period that matters to a buyer. And is preventing the measured engine wear all we need from oil? Perhaps another property matters to running the engine, and the brands differ on that property.
What we’re looking for
We're asked to weaken the conclusion that cheaper brands are the best buys. I'd first look for a longer-term difference, but evidence that the comparison leaves out another important function could also challenge the recommendation. The price comparison is persuasive only if the cheaper oils do well enough on the things that matter.
Working the answers
- A
Knowledgeable mechanics choosing cheaper oil for their own cars would tend to support the recommendation. It isn't proof that cheaper brands are always best, but it gives us no reason to reject the claimed value of those brands.
- B
Other reliable tests could be available without changing the comparison at all. Those tests might find that the cheaper oils perform just as well too. B says there are other ways to assess quality; it doesn't identify an overlooked property important to engine operation or give a result unfavorable to the cheaper brands.
- C
This applies to all motor oils. They could deteriorate at the same rate, leaving the cheaper oil just as effective for the relevant period. Although deterioration over time sounds related to our concern about longer distances, we need a reason it undermines the comparison between cheaper and more expensive oils.
- D
Which brand, and how expensive was it? The unusually long-lasting engines could have used a cheap brand, which would fit the recommendation. Frequent oil changes could matter too. Without a useful comparison between the brands under consideration, this example doesn't show that paying more buys better value.
- ECorrect
This addresses the second concern: reducing engine wear isn't the only property important to running the engine. The study's equality on that measure therefore doesn't settle the full comparison. A cheaper oil could protect against the measured wear yet perform worse on another important function. E doesn't prove that expensive oil is better, but it gives us a reason to question using this single result to declare the cheaper brands the best buys.
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