Q9 - Most lecturers who are effective teachers

[gregory.mortenson]

I narrowed the answer choices to A and C but then got stuck. I went with my gut and chose A, but I can't logically disprove C. Why is C wrong?

The only thing I can think of is the use of the word "very" in the line about noneccentric lecturers. Otherwise C seems to check out. What am I missing? Please help!

[dan]

I'll give you an analogous argument. See if you can apply it to the original:

Analogous argument:

Most children who attend the party are happy.
But, some unhappy children are at the party.
Every person at the party is an inspiration.

Answer (A): THUS, at least some inspirational people are happy (we know that for sure... there are children at the party who are both inspirational and happy).

Here's what (C) says:

THUS, some children who are not at the party are unhappy.

What do we know about children NOT at the party? Nothing! We know that "some unhappy children are at the party," but we don't know for sure if there are other unhappy children out in the world. After all, "some" could mean "all."

Hope that helps.

dan

[gregory.mortenson]

Makes perfect sense! The alternate example really helped me see the gap in logic. Thank you!

[rbolden]

I still have questions about answer choice C.

I thought that if you had two double negatives in a sentence such as, "Some lectures who are not effective teachers are not eccentric" you could take out the negatives and the sentence essentially means "Some lecturers who are effective are eccentric." And this last sentence would indeed be provable based on the passage.

Please explain why my reasoning is incorrect. I believe there was another LR question I came across were switching a double negative is what lead me to the correct answer. I know my reasoning is obviously wrong but I can't pinpoint why. Can you please explain this particular answer choice in detail and explain how sentences with double negatives generally come into play for inference questions?

Thanks.

[ManhattanPrepLSAT1]

Interesting question rbolden...

Unfortunately, taking out the negatives like that won't work when manipulating conditional logic on the LSAT.

If "~A some ~B"

that does NOT imply

that "A some B"

It's possible that the manipulation led you to the correct answer in a previous situation, but it's definitely not a manipulation permitted by logic.

Here's how I see this argument. There are three statements in the stimulus and since the subject of the statements are lecturers, I've left that term out of the notation. Leaving out the subject is a common way to simplify the logic.

1. ET most E
2. ~E some ET
3. ET ---> GC

From these three we can infer two things

Combining premises #1 and #3 we can infer E some GC

Combining premises #2 and #3 we can infer ~E some GC

The former is represented in answer choice (A). Remember it's okay to read a some statement backwards. If some A's are B's, then some B's are A's.

However, it's not permitted to read a most statement or an all statement backwards.

Does that help clear this one up?

[mcrittell]

However, it's not permitted to read a most statement or an all statement backwards.

Does that help clear this one up?

Can you expound on this please (I just want to cover my bases, as I'm new to this whole semi-conditional (?) setup thing)?

[timmydoeslsat]

Hey mcrittell, I just did this question. I always rely on diagramming for these types of questions. This is a place to use diagramming for sure.


Lecturers that are Effective Teachers MOST Eccentric

Lecturers that are Effective Teachers SOME Noneccentric

And we know that every effective teacher is a good communicator. This meas that our lecturers that are effective teachers qualifies us to say:

Lecturers that are Effective Teachers ---> GC

We can flip some statements.

A some B...can is exactly the same as saying B some A. No logical difference.

With most statements, we can switch them, but with an important distinction. The switching of a most statement makes it a some statement.

A most B....B some A

A most B can of course also be seen as A some B, as that is implied in a most statement.

An all statement such as every A is a B:

A ---> B

Can be seen as B some A.
---------------------------------------------------------

In this question, our idea of the effective teachers all being good communicators tests our ability as a test taker to show that this is something that must be true:

Our three statements:

1) A MOST B

2) A some ~B

3) A ---> C

We can combine 2 and 3 like so:

~B some A ---> C

So we know that there are ~B's that are C's.

We can combine 1 and 3 like so:

Flip the most to some...

B some A ---> C

So we know that there are B's that are C's.

[LSAT-Chang]

Would "some good communicators are noneccentric" also be a valid inference?

Since we know that SOME lecturers who are very effective teachers are noneccentric and we also know that EVERY effective teacher is a good communicator, there should be at least one lecturer who is a very effective teacher who is noneccentric, so at least one should be an effective teacher who is a good communicator that is noneccentric -- right?

But a question I had was.. can we combine a SOME + ALL statement, and infer SOME? So:

some As are Bs
all As are Cs
therefore, some Bs are Cs?

I wasn't sure since all we learn is "some+some" and "some+most" and "most+most" and we can only infer "some" from the last statement "most+most". Was just curious to know about this case, since it made sense to infer some good communicators being noneccentric.

Also, I got answer choice (A) by simply combining the two statements:

1) most lecturers who are effective teachers are eccentric
2) every effective teacher is a good communicator

since we have MOST + EVERY, there must be at least some overlap between "eccentric" and "good communicator". Since the correct answer choice (A) does infer an overlap between the two, I guess having a MOST statement and an EVERY statement allows us to infer SOME overlap, but not sure about SOME+ALL statements.. I'm assuming we could infer "SOME" from that as well. Any thoughts?

[timmydoeslsat]

Would "some good communicators are noneccentric" also be a valid inference?

But a question I had was.. can we combine a SOME + ALL statement, and infer SOME? So:

some As are Bs
all As are Cs
therefore, some Bs are Cs?

Yes, that is also a valid inference.

You can derive an inference with a some and an all statement as long as one of the variables in the some statement IS THE SUFFICIENT CONDITION.

Example:

A some B

B ---> X

A some B ---> X


We know that every B is X. We know that some A's are B's, thus we know that some A's are X's as well.


A some B

X ---> B

X ---> B some A

We know that every X is a B. We know that some B's are A's. But we do not know that there is an overlap.

XXXXXX
BBBBBBBBBBBBBBBBB
-----------------AA


Another way to make this an automatic response is this:

If you have a modifier "some, most" and we can place that before a conditional all arrow, we will be able to make inferences with that variable and what is on the other side.

When we have a some or most statement after a conditional all arrow, we do not know anything.

[sumukh09]

I always have trouble with these types of questions; would you say the key in answering most of these types of questions is to recognize that some statements are reversible? Is this what the test makers are testing?

[aprilhu33]

I don't get why D is incorrect. I see now why C is incorrect and why A is correct but D also sounds legit to me. Can someone explain please? Thanks!

[doug.feng]

I don't get why D is incorrect. I see now why C is incorrect and why A is correct but D also sounds legit to me. Can someone explain please? Thanks!

I believe the problem with answer choice (D) is two things:

1. The slight mismatch from "effective teachers" to "good communicators," which is NOT inferable within the information provided in the rest of the answer choice.

2. Reversed logic (good communicators -m-> eccentric). If we wanted to give this choice the benefit of the doubt with the slight mismatch noted before, the answer choice goes in the wrong direction. To make this question correct, it should have went from effective teachers -m-> eccentric -s-> good communicators.

[ohthatpatrick]

Let me see if this is where you confusion is coming from:

We're given,
Effective teacher --> Good communicator.

So maybe you were thinking the first sentence,
"Most L's who are effective teachers are eccentric"
could be turned into
"Most L's who are good communicators are eccentric"

After all, if you're an effective teacher, I know you're a good communicator, so why can't we just swap in one label for the other?

Here's the problem, via analogy:
US President --> Male

Given the statement:
"Most US Presidents have lived in the White House
can I turn that into
"Most males have lived in the White House"
??

Of course not.

Even though every US president has been a male, making a statement about "most US Presidents" is VERY different from making a statement about "most males".

Similarly, making a statement about "most L's who are effective teachers" is very different from a statement about "most L's who are good communicators".

Or if you prefer numerical examples ...
there are 100 L's who are effective teachers.
51 of them are eccentric, 49 are noneccentric.

(All 100 of those teachers are good communicators, since they're effective)

Meanwhile, there could be 1000 L's who are good communicators.
Can we really prove that 501+ of them are eccentric?

We don't even know how many of them are effective teachers. We need 51 of them at a minimum to be effective (because we already stated that 51 ppl are effective/eccentric/good communic). But that's it. The other 949 are up for grabs. No reason they have to be eccentric.

Let me know if you're still confused.

[Amontillado]

Hi, I got the question right, but still very confused with the first sentence: "most lecturers who are effective teachers are eccentric." Are we assuming that "lecturer=teachers"? If yes, then I have no problem with it anymore, simply write it as "EL--most-->E". However, if "effective teacher" is a much larger group than lectures, we can only infer "ET<--some-->E". Still can get the right answer A.

What do you guys think?

[aaronvoldman]

Hi, I got the question right, but still very confused with the first sentence: "most lecturers who are effective teachers are eccentric." Are we assuming that "lecturer=teachers"? If yes, then I have no problem with it anymore, simply write it as "EL--most-->E". However, if "effective teacher" is a much larger group than lectures, we can only infer "ET<--some-->E".
What do you guys think?

Amonitllado, Even if it is hypothetically true that "Effective Teacher" is a much larger group than lecturers, we can still infer

ET (that are lecturers) MOST Ecc.

The text gives us that which relates to the specific smaller group, those that are both Lecturers and are Effective teachers. It is this group that is our focus. Whether or not the group is just a portion of the larger group, we know what is true based of the smaller group of those that are both lecturers and effective teachers.

As noted above in this discussion, we can drop the subject (Lecturers) in our notation, since that which leads to our inference is something that is true of all of those who have a particular distinction (of being effective teachers), including those who are lecturers + effective teachers.