PrepTest 120

Incorrect. The conclusion is about whether the books are useless as reference—that's a determination outside of whether they're consulted. A writer can consult a book that ends up being useless and fail to consult a useful book!

Incorrect. There's no claim that an author doesn't mistakenly think that a sentence is ungrammatical . . . though this sentence sure is at least grammatically problematic!

Incorrect. The conclusion limits itself to whether these books are useful to authors, so this answer about non-authors is out of scope.

Incorrect. The conclusion limits itself to whether these books are useful as reference sources, so this answer is out of scope.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
Author thinks sentence is ungrammatical → Won't write it so doesn't need a grammar book
Author thinks sentence is grammatical → Won't consult a grammar book
Therefore - Grammar books are useless

Answer Anticipation:
There are two elements we can focus on in this stimulus that point us to the error in reasoning.

The first is the extreme conclusion—it calls something "useless." While it does narrow it down to being useless as a reference for writers, that means if a single writer has a single use for a grammar book as a reference, the conclusion doesn't hold. That's a tall order for the premises to support, so do they?

This is where the second element comes into play—it brings up two groups of writers and shows that they both lead to the same conclusion. For each of the mentioned groups, grammar books do serve no useful reference purpose. When an argument follows this pattern—it splits up a group and shows that the subgroups all end up at the same conclusion—we need to consider if those groups are comprehensive. In other words, do they represent all writers?

The answer is no, they don't. There are writers out there who aren't sure whether or not a sentence they're writing is grammatical, and the grammar books may serve as a valuable reference for them. Let's find an answer pointing out that it ignores a key subgroup in reaching the conclusion.

Answer Explanation:
This answer highlights that the premises don't address all subgroups of situations where authors are considering the grammar of a sentence. While they'll sometimes be sure the sentence is or isn't grammatical, there are other times—as this answer highlights—where they're uncertain. In those cases, the books could be useful.

Key Takeaway:
Whenever an argument starts splitting a group into subgroups to show the outcomes, consider whether the subgroups are comprehensive. If they're not, then the argument is ignoring possibilities, making it flawed.