What to Know About Sufficient and Necessary Assumption LSAT Questions

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

These question types are tricky, and also appear frequently in the LSAT 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 Logical Reasoning 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.

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 model? What if Suzy is actively in her dating era? Although those factors might improve Frank’s chances in real life, neither of them is enough to make the main 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. If Frank gets the phone number of every woman he ever talks to at a bar, then our argument is actually airtight. There is no wiggle room at all for an alternative outcome, and that’s what we want on a sufficient assumption question.

Luckily, the lack of flexibility on a sufficient assumption question actually makes them quite easy to anticipate! You would expect your answer choice to conform to the following conditional structure: “If [evidence] then [conclusion]”.

So in this case, that shorthand might look like “If Frank talking / if woman at this bar / if woman in question is Suzy … then Suzy gives number to Frank.” Using this anticipation, another potential right answer might be “If Suzy talks to someone at a bar, she will give them her number”.

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Necessary Assumptions

A necessary assumption is an assumption that needs to be true in order for the main conclusion to be possible.

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 an interesting guy— Suzy could give him her digits either way. So, what types of things are necessary? Well, that Suzy has a phone number, for one.

What absolutely has to be true, and if it weren’t, the argument would fall apart? These questions are not as formulaic as sufficient assumption questions. But luckily, there’s an easy way to test if a correct answer choice is indeed necessary! Just ask yourself, is there any way the argument could work if it’s false? In our case, if we considered what would happen if Suzy DIDN’T have a phone number, well the argument would be destroyed, right? There’s no way you can get to your conclusion!

Spotting the Differences

Necessary and sufficient assumption questions are very different, and – although this might sound self-evident to the point of seeming ridiculous – it’s important to know which question type 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 each question type. In case you’re a student finding 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.

Final Thoughts

Sufficient and necessary assumption questions may seem confusing at first, but with our LSAT prep, you’ll get better at identifying the different types of assumptions and anticipating what the correct answer choice might be.

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