Logical reasoning PrepTest 131 · Section 2 · Question 20

Incorrect. There's no indication of how many samples were tested, so there's no basis to say that it was too small of a sample.

Incorrect. This is a great . . . trap answer! However, the argument doesn't assume that there are equal chances of picking a defective and non-defective item, but that the items selected are representative. This answer would suggest that there should have been a 50-50 split since each "draw" of an item had an equal chance of being defective and nondefective.

Incorrect. Even if the defective items are localized to a few sites, that could drive the rate above the contracted-for level.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
We tested samples from our field inspectors and found 20% defective. Since the contract calls for defects to be under 5%, the supplier of the samples has violated the contract.

Answer Anticipation:The argument uses the word "samples" several times, telling us we should start by considering a sampling flaw. Is there a reason to believe that the samples in question aren't representative of the overall items manufactured in that they're more likely to be defective? Well, they were sent in by the field inspectors—it's not unreasonable to think that the samples sent in to be looked at by the lab are the ones that seemed off. If these submitted samples were more likely to be defective than the average item, then the sample is skewed and that 20% error rate might be much higher than the actual rate.

Answer Explanation:
This answer (finally) correctly describes the sampling flaw in the stimulus. The field inspectors are sending in samples, and for the error rate in the sample to match the error rate overall, it must be the case that the selected samples are random—that there isn't a tendency to select items that look defective. If the inspectors do tend to send in only items they suspect of defects (as this answer highlights), then the sample is skewed.

Key Takeaway:
Don't ignore language that hints at specific flaws when they show up in a stimulus! Here, the LSAT gift-wrapped the sampling flaw for you by using that word several times.

The difficulty here comes from the three answer choices that are all in the area of a sampling flaw—especially that trap answer choice (B). When you're choosing between two answers choices that are so close together, compare them to the stimulus and each other to find any relevant differences, and make your call from there. And DON'T get hung up at that point—if you don't see which answer is right after comparing them to the stimulus and each other, you probably won't with an extra minute or two and you need that time for other questions. Make your call and move on.

Incorrect. The number of times that the inspectors visited each site is irrelevant as long as they manage to grab a representative sample of items for defect testing.