PrepTest 109

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
There are hundreds of Sturdimade drivers who have driven their cars over 100,000 miles, so Sturdimades are reliable.

Answer Anticipation:
The conclusion here draws its conclusion about the reliability of Sturdimade cars by looking at the owners who have Sturdimades that have lasted for over 100,000 miles. Those cars are certainly reliable, but there's no indication that they're representative of all Sturdimades.

Maybe millions of Sturdimades are sold every year, and only hundreds of them have made it to 100,000 miles. If that's the case, then they're not looking all that reliable!

The short version? This argument is based on a sample of a few hundred who signed up for a club designed to prove the conclusion, and that sample is not shown to be large enough to draw the conclusion and it's more or less guaranteed to not be representative.

Answer Explanation:
This answer highlights the Sampling flaw present in the stimulus. It bases a conclusion about reliability on those who signed up for a club requiring that your Sturdimade be reliable. That's a skewed sample and possibly too small to be reliable, so this answer is correct.

Key Takeaway:
Whenever a sample is "opt in"—here, by signing up for the "long distance" club—it's very likely unrepresentative. A sample generally needs to be random in order to be representative.

Incorrect. The conclusion is that Sturdimades are reliable, and the evidence is that at least a few hundred have been driven over 100,000 miles. That's not simply restating the premise—it's using a sample to back it up (even if that sample is skewed).

Incorrect. "Reliability" is the key term here, and there's no ambiguity—it is used to mean that the car will hold up after being driven for many, many miles.

Incorrect. Sturdimades that last a long time go to support a conclusion about their reliability.

Incorrect. The argument doesn't bring up popular opinion about the reliability of Sturdimades—it brings up specific examples of the car lasting a long time.