Why is an assumption necessary for a conclusion

[cavitand]

Hello, my first post here.

since we need an assumption to get a conclusion (assumption----> conclusion), we should say an assumption is sufficient for a conclusion. But, Manhattan says it is necessary. I am confused about this.

Thanks for helping out.

[DanLerman]

Cavitand--

Excellent question--

In short... an assumption alone is NOT sufficient to draw a conclusion. On the GMAT, there will always be at least one PREMISE, or piece of proof, that when combined with an assumption, allows the author to draw a conclusion.

That's a bit vague, so let's look at a simple GMAT-like argument.

Michael is tall. Therefore, Michael will play in the NBA.

The CONCLUSION, or the biggest claim that the author makes, is the second sentence. And the PREMISE, or proof that the author uses to make that conclusion, is the first sentence. Nice, simple, clean cut.

Now to make the logical jump from Michael is tall --> Michael will play in the NBA, the author needs to make ASSUMPTION(S). On this test, assumptions are never actually stated. And remember, we can't just say that the author of this argument is delusional... so let's think: what must the author think is true in his/her head in order arrive at the stated conclusion?

In this case, there are a bunch of assumptions the author makes to arrive at 'Michael will play in the NBA'-- to name a few, the author is assuming 'Michael is athletic,' and that 'Michael WANTS to play in the NBA,' and that 'Michael's last name must be Jordan' (just kidding). But these assumptions should jump out at you necessary pieces that connect the logical gap between the author's premise(s) and the author's conclusion. Without this logical connection, there is NO WAY the author could make such an outrageous claim! Therefore these assumptions are NECESSARY.

Now your question makes me think that you might be reading these questions backwards-- start by identifying the conclusion (about 90% of CR questions have conclusions in them, the other 10% ask YOU to draw a conclusion). Then, based on the evidence that you are given (premises), ask yourself: what information is required to connect the gap? These will be the assumptions!

And here is a big, take home message: (!!!)

EVERY CONCLUSION YOU SEE ON THE GMAT WILL REQUIRE SOME SORT OF ASSUMPTION!

Let's take one more argument that looks pretty good at first glance:

Viv got 4 hours of sleep last night. Therefore, Viv will be tired today.

Now, this actually seems like a pretty decent argument. But if I really nitpick, I can think of certain assumptions (even though they are small!) that the author is making. For example, he/she is assuming that 'Viv needs more than 4 hours of sleep to function normally (maybe Viv can function on 4 hours!),' and that 'Viv is not on some sort of stimulant today,' and so on.

As you get more experience with CR questions, your brain will automatically start to see assumptions popping up in every argument. You'll even start to see them in the arguments that people make day-to-day. This test will change your life!

[tim]

I'll jump in here to point out that although our materials make a big deal out of necessary assumptions - and the majority of assumptions questions focus on necessary assumptions - some questions do in fact ask for assumptions that are sufficient rather than necessary. The key is to learn how to tell the difference, because this will make a huge difference in figuring out what the correct answer is. Keep in mind though that sufficient assumption questions are quite rare, and the question you brought up definitely referred to necessary assumptions (you even wrote the word "need"!).

[cavitand]

Thanks a lot

[tim]

:)