This technique is designed to help you see which assumptions are necessary to an argument. The idea is that if the assumption is negated, the argument should no longer make sense. Let's look at an example:
Tigers are carnivores.
Elephants are herbivores.
Therefore tigers are more dangerous than elephants.
The argument depends on which of the following assumptions?
A) Some herbivores are highly dangerous.
B) Carnivores are more dangerous than herbivores.
C) Tigers are the most dangerous animal in existence.
D) Elephants are not dangerous.
E) Some carnivores are more dangerous than others.
Looking at these choices, we might conclude that B,C, and D all support the argument. What happens when we negate these choices?
A) The negation is "No herbivores are highly dangerous." Negating this answer would actually strengthen our argument. We can cross this one out. (Note that it's not unusual to see this sort of tempting wrong-direction choice as answer A).
B) The negation is "Carnivores are not more dangerous than herbivores." If that's true, then we have no reason to conclude that tigers are more dangerous than elephants. B is a necessary assumption.
C) The negation is "Tigers are not the most dangerous animal in existence." If that is true, tigers still might be more dangerous than elephants. This choice would make a great "strengthen" answer, but it is not necessary to validate the conclusion. We can cross it out.
D) The negation is "Elephants are dangerous." This seems to go against our argument, but it doesn't tell us how dangerous they are in comparison to tigers, so it doesn't knock out our conclusion. Again, this would help to strengthen the argument, but it is not necessary in order to reach the conclusion. D is out.
E) The negation is "All carnivores are equally dangerous." Okay, but are they more dangerous than herbivores (specifically elephants)? We don't know, so this has no effect on pour argument. This is a classic Variation Trap. The GMAT loves to throw this kind of thing in to make us say "Hmm . . . I wonder if that makes a difference." It makes no difference. If B is true, then we know tigers are more dangerous than elephants. We just don't know if they are more dangerous than other carnivores. If B is not true, then this information doesn't matter, as we have no basis for comparison.
In this case, you might have seen right away that B filled the logic gap, in which case you wouldn't need to run these negations. However, practicing them might prove helpful for those tricky situations when you're down to 2 choices and you just can't see what makes one wrong and the other right. Try doing this when you're reviewing your work, and you should find that you get much faster and more effective at all assumption-based arguments (Assumption, Strengthen, Weaken, Flaw, Evaluate the Argument).
I hope this helps!
Dmitry Farber | Manhattan GMAT Instructor | New York
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