PrepTest 131

Incorrect. Without information on how very high speed limits factor into accident or injury rates, this answer is out of scope.

Incorrect. The stimulus never talks about the drivers, always relating the rates to the cars that they're driving, so this answer doesn't confound the information.

Incorrect. This would still allow for 5,000 people to make up the sample of people in large and small cars, which is still more than enough to form a valid conclusion.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Weaken

Stimulus Summary:
Sample: 10,000 people who were in accidents
Large automobile—Low percentage injured
Small automobile—High percentage injured
Conclusion: You're less likely to be injured in an auto accident if you drive a large car than a small car

Answer Anticipation:
The argument starts with a sample, so we should start by considering a sampling flaw. The sample is definitely large enough—10,000 is many times the number that would be needed to draw a valid conclusion. Is it representative?

Nope! The conclusion is about all drivers, but the argument looked only at those who were involved in car accidents. While someone who is in an accident while driving a large car is much less likely to be injured than someone who is in an accident while driving a small car, the overall chance of being injured in an accident (what the conclusion is about) also takes into account the odds of being in an accident in the first place. If large cars are much, much more likely to be in an accident, then even though they're safer in the accident, you might be more likely to be injured due to the increased accident rate.

To put it another way, let's say 1/100 people in large cars are injured in accidents, compared to 1/10 people in small cars. If, however, large cars are 1,000x more likely to be in a car accident, then someone in a large car will be more likely, overall, to be injured, when comparing both the accident rate and the injury rate.

Let's find an answer that brings up the possibility that large cars are more likely to get into accidents in the first place, thus suggesting that there might be more injuries for those driving these larger vehicles when that's taken into account.

Answer Explanation:
This answer highlights the issue we found with the sample, and the shift from looking at people who were in accidents instead of everyone who drove a large/small car. If a large automobile is much more likely to get into an accident, then someone driving one might be more likely to be injured in an accident even if they're safer once in that accident.

Key Takeaway:
Any time a survey or sample is drawn from a specific group—here, those who were involved in accidents—it can't be used to justify a conclusion about the overall population. Whenever the sample is qualified in any way, be very wary of it.

Incorrect. Since the conclusion is comparative, the overall injured-in-accident rate doesn't matter, as long as it's big enough for there to be a statistically significant difference for cars of different sizes. This answer also just addresses those in accidents, not all drivers, falling into the same gap present in the stimulus.