Sufficient and Necessary Conditions Questions

[dirrty_30]

I was doing the suffiicient/necessary conditions exercises from the LSAT Course book and had a few questions. Can someone please tell me where my thought process is off in the following questions?

(Forgive me I can't figure out how to strike through text)

16. "No one has the right to address the council except for the chairman"

Chairman --> Right to address council

No right to address council --> Not the chairman

17. "I'll be here until Mom picks me up."

I said: be here--> Mom does not pickup

Mom picks up --> Won't be here

18. "If you're not happy, you married Robert."

Marry Robert --> Not happy

Happy --> Not married to Robert


Thanks!

[Jonathan Evans]

Hey Dirty,

It's excellent that you're spending the necessary (no pun intended) time working on these fundamental concepts. Your efforts will be rewarded in a fuller understanding of the mechanics and underlying logic that governs LSAT questions, and consequently you will achieve a better score.

Remember that the necessary condition is a minimum requirement that must be met for something else possibly to be true.

The sufficient condition is something that by itself guarantees the truth of something else, but there could be multiple different sufficient conditions that guarantee an outcome.

Let's go through your questions:

16. "No one has the right to address the council except for the chairman"

Chairman --> Right to address council

No right to address council --> Not the chairman

Who do you have to be to address the council? The chairman. That's necessary. The necessary requirement goes on the right hand side of the arrow.

RtAC C
~C ~RtAC

17. "I'll be here until Mom picks me up."

I said: be here--> Mom does not pickup

Mom picks up --> Won't be here

Again, what's the thing that has to happen? Mom has to pick me up for me not to be here anymore. Necessary. Right hand side.

~H M
~M H

18. "If you're not happy, you married Robert."

Marry Robert --> Not happy

Happy --> Not married to Robert

This one is straight up "if...then." "if" introduces a sufficient condition. What by itself here is enough to know something else for sure? Being unhappy. That is enough to know for sure that you married Robert. What if you didn't marry Robert? Then you're happy. You're not not happy.

~H MR
~MR H

You have to revise your thinking out of "real world" uses of language, ways that we would introduce assumptions into these statements. Accept that sometimes the structure of conditional statements will lead to outcomes that are at best counter-intuitive, even stupid at times! (a couple LR questions come to mind). However, you must rely on these concepts of "what is required" and "what is enough/a guarantee" to identify necessary and sufficient conditions. Look for indicator words, familiarize yourself with the "Unless Equation," but above all else try to zero in on these concepts of necessary and sufficient. I hope this helps.

[dirrty_30]

Would making flashcards of indicator words help? I've been considering that

[Jonathan Evans]

Yes, certainly. that could be helpful, but flashcards are no substitute for zeroing in on these concepts.

Necessary means a requirement.

Sufficient means a guarantee.

If you notice something that looks like a requirement, whether from indicator words or just from syntax, take note of it immediately and ask what it is required for. Operate under the assumption that requirements are NOT also guarantees.

"What it is required for" "requirement"

If you notice something that looks like a guarantee, something that looks like it is enough to force something else to be true, take note of that immediately and ask what it guarantees. As above, operate under the assumption that guarantees are not requirements.

"Guarantee" "what it guarantees"

Consider the following examples:

1) To get a home loan, you need at least to provide a W2, employment history, a bank statement, and credit report. Every one of the requirements is necessary but none of them by itself is sufficient to guarantee getting a loan. In fact, even if you were to have all these things, they still might not be sufficient. You might also have to have a signed contract on your house, etc. This example is meant to illustrate what it means for something to be necessary. It is a requirement but not a guarantee.

2) The bases are loaded and the count is full. The Expos are at bat. What would it take for the Expos to score a run. Well, the pitcher could throw four balls. That would be enough to walk the batter and score a run. The batter could hit a line drive double. That would be enough to score a run. The batter could hit a home run. That would be enough to score a run. The pitcher could throw a wild pitch and hit the batter, causing a walk, scoring a run. Any of these events would be sufficient to guarantee that a run is scored, but none of them is a requirement for a run to be scored. This example is meant to illustrate a sufficient condition. It is a guarantee but not a requirement.

Now, when you encounter things that look like necessary or sufficient conditions on the test, make a note of them immediately. These conditional situations are the source of both the credited responses and many, many attractive wrong answers. Master these concepts; remember to adhere to very strict definitions of each; do not let everyday assumptions muddy up your thinking. Feel free to make notecards as a way to help you identify these situations when they occur, but try to rely on your innate grasp of these concepts when you are working through stimuli. That way you will fall into fewer traps.