L.R. - Necessary vs. Sufficient Assumption Questions

[neri-lozano]

The "find the cap" in the conclusion was very helpful. I want ti knw how to attack sufficient assumptions, how do they differ from necessary assumption ones? What do I need to look for, do I approach them the same way? Will the sufficient assumption "must follow" just as the necessary one? Will the right answer also have strong/mild language have strong /mild language if conclusion does.

Also, how do I approach the "if" assumptions, the ones that say, "The following conclusion can be drawn IF which one of the following is assumed? Are these in the sufficient category? Its very confusing because it sounds like inference/assumption question or do they just put the "conclusion" part to throw us off?

In general I can find the assumptions without to much problem. The issues arise because i don't know how attack the answer choices. What am I suppose to be looking for in the Necessary and Sufficient, do I approach them the same way or not?

Thank You

[ManhattanPrepLSAT1]

Those are some very large questions you're asking, but let me try to summarize the following: distinguishing Sufficient Assumptions from Necessary Assumptions, your task in each one, some common argument structures in each, and characteristics of correct/incorrect answers.

Distinguishing the two forms of Assumption questions:
If you see an assumption question, look for language cues implying sufficiency such as "if" "allows" "enables" and "suffices" or words implying necessity such as "only if" "requires" "depends on" and "must." Sufficiency triggers tell you it's a Sufficient Assumption, necessity triggers Necessary Assumptions.

Examples
- Which one of the following assumptions would allow the conclusion above to be properly drawn? "allows" = Sufficient Assumption
- The conclusion above is properly drawn only if which one of the following is assumed? "only if" = Necessary Assumption

If you do not see language implying sufficiency or necessity, the Assumption question is automatically a Necessary Assumption.

Example
The argument assumes which one of the following?

Your Task
On Sufficient Assumptions, your task is to find an assumption that would guarantee the truth of the conclusion.
On Necessary Assumptions, your task is to find an assumption that needs to be true in order for the argument's reasoning to even have a chance of being true, but may not be enough to guarantee the conclusion.

Sufficient Assumptions mostly work by asking you to find the missing piece of the puzzle in a chain of conditional reasoning.

Necessary Assumptions work in that manner sometimes, but also works by asking you to defend the argument from something that would be devastating to the argument's reasoning (Ex: possibly ruling out a competing explanation to one concluded by the author).

On Sufficient Assumptions, strong and precise language is a valued characteristic, whereas on Necessary Assumption questions such language can get an answer choice into trouble. This leads to the preference for weak or vague language on Necessary Assumptions.

[Kristinav10]

I’m confused I thought the necessary assumption guarantees the argument and the sufficient assumption is “enough” for the argument to be true. I think the second part where you discuss the tasks for necessary and sufficient conditions should be switched

[ohthatpatrick]

No, Necessary Assumptions don't guarantee the truth of the conclusion.

"Guarantee the conclusion" and "enough for the argument to be true" mean the same thing.

ALL SYNONYMOUS
conclusion follows logically
conclusion is properly drawn / properly inferred
airtight argument
logically valid
evidence is enough to imply / guarantee / ensure conclusion

Here's the simplest way to remember the difference between the two types of questions:

SUFF ASSUMP:
which answer, if added to the evidence, would allow me to 100% derive the conclusion?

NEC ASSUMP:
which answer, if negated, would most weaken the argument?


Consider this argument:
Jane lives in the United States and spends all her time in the same state. Therefore, she must live in Kansas.

SUFF ASSUMP:
If you spend all your time in the same state, you live in Kansas.
if I add this rule to the evidence, which said that Jane spends all her time in the same state, then I can 100% derive that Jane lives in Kansas.


NEC ASSUMP:
People who spend all their time in one state do not all live in California.
[i]If I negate this, it says that people who spend all their time in one state DO all live in California. That would mean that Jane lives in California, so this badly weakens (if not refutes) the original argument.


With a Sufficient Assumption, there are very few options for a correct answer (usually it will feel like there's only one possible correct answer, because there's usually only one missing link/idea in the arguments we're presented with for Sufficient Assumption).

With Necessary Assumptions, there are usually many possibilities for a correct answer. I could re-write the one I just provided for 48 other states (f.e. "People who spend all their time in one state do not all live in New Jersey"), and EACH of those assumptions is necessary, but none of them on their own could guarantee the truth of the conclusion.


Hope this helps.

[LukeM22]

Just wanted to bump this as I came across a question that made me think of this, PT 79 Section 4 Q6

It seems like some necessary assumptions stimuli are structured exactly the same as a sufficient assumption question—the only difference being that the answer, as you mentioned, is “weaker” in the sense that doesn’t guarantee the truth of the conclusion, but negating it kills/weakens the argument. My question is this: if the stimuli/paragraph for an NA question pretty much looks exactly the same as an SA question, what would materially differ in the approach we should take besides looking for the right language in the answer choice?

Thanks again for answering all my questions!

[ohthatpatrick]

I'm not sure what about that question reminds you of Sufficient Assumption (more so than other arguments).

Even if we beefed up the answer choice and said "Giving ample food but little exercise is ALWAYS unhealthy", that's still not a sufficient assumption.

Can we derive the conclusion that "these factors can skew the results of research using animals"?

Not yet. We still have a NEW GUY in the conclusion: "what's the rule for knowing whether or not results have been skewed?"

We'd also need a rule that says:
"If a study relies on a false assumption, then it's results can be skewed."

------------------------

I think you're just reacting to the fact that Necessary Assumption tends to feel like one of two different things:

- a somewhat messy story, often involving a Plan or Causal Interpretation, and the correct answer rules out an objection or alternative explanation.

- a language math paragraph, with repeating symbols (possibly some conditional logic), and the correct answer supplies a missing link or idea.

That 2nd form will feel like Sufficient Assumption, since 95% of Sufficient Assumption questions are of that form.

And both questions are the same in the sense that we should pay attention to New Guys in the conclusion and listen for missing links or ideas.

But they are different in the sense that when I read the conclusion to Q6 on a Sufficient Assumption question, my mindset becomes, "Okay, I need a rule that says ... if such-and-such is true, then the results are skewed".

When I read it on Necessary Assumption, that's ONE possibility, but I would still be looking elsewhere in the argument for possible assumptions / objections.