One of the types of critical reasoning questions you'll see on the GMAT is the assumption. An assumption bridges the gap between an argument's evidence and conclusion. It's a piece of support that isn't explicitly stated but that is required for the conclusion to remain valid. When a question asks you to find an author's assumption, it's asking you to find the statement without which the argument falls apart.
In order to test whether a statement is necessarily assumed by an author, we can employ the Denial Test. Simply deny or negate the statement and see if the argument falls apart. If it does, that choice is a necessary assumption. If, on the other hand, the argument is unaffected, the choice is wrong.
Consider the following example:
Allyson plays volleyball for Central High School. Therefore, Allyson must be over six feet tall.
You should recognize the second sentence as the conclusion and the first sentence as the evidence for it. But is the argument complete? Obviously not. The piece that's missing is the assumption, and you could probably rephrase this one pretty easily:
All volleyball players for Central High School are over six feet tall.
Now, let's use the Denial Test. What if it's not true that all volleyball players for Central High School are over six feet tall? Can we still logically conclude that Allyson must be taller than six feet? No, we can't. Sure, it's possible that she is, but it's also possible that she's not. By denying the statement, then, the argument falls to pieces; it's simply no longer valid. And that's our conclusive proof that the statement above is a necessary assumption of this argument.
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