Sufficient and Necessary Assumption Questions, Demystified

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Sufficient and Necessary Assumption questions are tough. Don’t get discouraged.

Halloween may be over, but spookiness still lurks around the corner for LSAT-studiers who are just getting to sufficient and necessary assumption questions. These question types are tricky, and also appear frequently in the Logical Reasoning section, so it’s important to have a firm handle on what each type of assumption means. If all you know is that these question types are sufficient to give you a headache, read on!

Sufficient Assumptions

A sufficient assumption is an assumption that, if true, would make the whole argument totally valid. A necessary assumption is an assumption that needs to be true in order for the conclusion to be possible.
For example: Frank is talking to Suzy at a bar. Therefore, Frank will get Suzy’s phone number.

What is sufficient to prove the conclusion in this case? What if Frank is a male underwear model? What if Suzy has had eight shots of tequila? Although those factors might improve Frank’s chances in real life, neither of them are enough to make the conclusion 100% true. A sufficient assumption is going to be something so strong that it actually sounds kinda silly — for instance, that Frank gets the phone number of every woman he ever talks to at a bar.

Necessary Assumptions

Now let’s use the same example to discuss what assumptions are necessary. Again, it’s not necessary that Frank is some kind of Adonis, or even that he’s marginally good looking — Suzy could give him her digits either way. So, what types of things are necessary? Well, that Suzy has a phone number, for one — otherwise it’s going to be mighty hard for her to give that number to Frank.

Spotting the Differences

Sufficient and necessary assumptions are very different, and – although this might sound self-evident to the point of seeming ridiculous – it’s important to know which type of assumption the LSAT is asking you to supply. That said, I remember from my own studies that I was persistently confused about how to tell the difference between the two question types. In case you find yourself in a similar boat, here’s a run-down on the difference.

Here are some examples of how the LSAT might ask about sufficient assumptions:

Which one of the following, if assumed, would allow the conclusion to be properly drawn?

The conclusion of the argument follows logically if which one of the following is assumed?

In these prompts, the key is to notice that the test is asking you to make the argument valid. If you’re supplying an assumption that allows the conclusion to be properly drawn, that means you’re trying to find a sufficient assumption.
And here’s how it might look when the LSAT wants you to find the necessary assumption:

Which one of the following is an assumption on which the argument depends?

Which one of the following is an assumption required by the argument?

In these cases, the prompts are giving you synonyms for the word “necessary,” so you know they’re asking which assumptions are necessary – or required – in order for the conclusion to be possible.

Sufficient and necessary assumption questions may seem spooky at first, but with practice, you’ll get better at identifying the different types of assumptions and anticipating what the correct answer might be. We’ve got plenty of other discussions about these question types on the blog, so check ‘em out and let us know if you still have questions!

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