LSAT 131 | Section 2 | Logical Reasoning: Q20

LR Question 20 Explanation

QUESTION TEXT: Quality control investigator: Upon testing samples

QUESTION TYPE: Flawed Reasoning

CONCLUSION: The manufacturer violated their contract.

REASONING: The manufacturer must produce fewer than 5% defective items. 20% of the samples selected for testing were defective.

ANALYSIS: The quality control inspector ignores an obvious possibility: maybe the field inspectors selected samples that looked like they might be flawed.

If that’s true, then we’d expect a high flaw rate in those samples. The overall flaw rate might still be below 5%.

___________

1. The stimulus actually doesn’t say what the sample size was, so there’s no basis for this.
2. The inspector didn’t say this! If inspectors were equally likely to choose defective and non-defective, then the error rate would have been 50%.
3. The samples came from “various manufacturing locations”. We have no indication that this was an unrepresentative sample of manufacturing sites. On the LSAT you basically assume reliableness of a sample unless there is specific evidence that the sample is limited.
   It’s of course possible that, within all factories, a few manufacturing sites produce most of the errors. But that could still be a violation! It drives up the overall error rate, after all.
4. CORRECT. If this is true, then the samples aren’t representative of the overall error rate.
5. The number of visits isn’t the right measure. We care about the number of defects produced.