Do you diagram contrapositives? (POLL) Forum

[letsdoit1982]

Is it a waste of time to diagram contrapositives? I feel like it's unnecessary, but it seems like a lot of people on TLS suggest diagramming them.

[ravens20]

I found diagramming contrapositives to be useful for logic games..they only take a few seconds to write down and can help you spot something you might have otherwise missed or at least taken longer to see.

It is especially useful to diagram contrapositives when you have long If/Then chains in IN/OUT games (which seem to be all the rage lately), especially when you have an AND/OR clause.
-Say you have some rule like: G -> A -> /T -> R AND Q (where / equals "not")

CONTRA: /R OR /Q -> T -> /A -> /G
If a question asks for the implications of Q being out then you have everything laid out for you without having to think too much at all. You know that A must be out, for instance, and you can deduce this in a matter of seconds.

[hellojd]

^^ I second the above post. I've wasted 30secs - 1 minute on certain games just because I didn't have the contrapositives diagrammed. It's really just an extra 3 seconds that can save you time in the long run.

The only trouble is if you get a very inference-heavy game, and then putting down every single contra can get messy if there's limited space.

[maks25]

I usually do but for some reason didn't on one game on the Feb LSAT, I lost almost 2 minutes because of it .

[thunder 85]

I generally do so on LG and on any formal logic questions that fall between questions 16-22 on the LR section. Its simply prudent.

The only exception is there are certain Selection and Distribution Games where is better to translate the rule than to write the formal logic. Negative X--> Positive Y translates to X/Y or Both (depending on game). Those you should just learn.

[DoctorNick189]

Do it.

[hellojd]

I generally do so on LG and on any formal logic questions that fall between questions 16-22 on the LR section. Its simply prudent.

The only exception is there are certain Selection and Distribution Games where is better to translate the rule than to write the formal logic. Negative X--> Positive Y translates to X/Y or Both (depending on game). Those you should just learn.

Yup, that and the Positive x --> Negative y turns into x<--/-->y

[letsdoit1982]

^^ I second the above post. I've wasted 30secs - 1 minute on certain games just because I didn't have the contrapositives diagrammed. It's really just an extra 3 seconds that can save you time in the long run.

The only trouble is if you get a very inference-heavy game, and then putting down every single contra can get messy if there's limited space.

This is one major problem I have with contrapositives. In and out games take forever, and when I diagram the contrapositive of each conditional statement, I usually run out of time.

[keg411]

Yes, yes, yes! Helps so much with inferences and speed as typically one question can be answered quickly with the help of a quick glance at the contrapositive!

BTW, figuring out contrapositives made in/out games click for me. I was so uncomfortable with them for a very, very long time.

[Knock]

Yes, yes, yes! Helps so much with inferences and speed as typically one question can be answered quickly with the help of a quick glance at the contrapositive!

BTW, figuring out contrapositives made in/out games click for me. I was so uncomfortable with them for a very, very long time.

What was it that made it click? I'm still a little iffy with contrapositives.

Are you supposed to take the contrapositive for every if statement? or only ones that start with a negative, like /R -> T so contrapositive that /T -> R. or if its X -> /Y should I also contrapositive Y -> /X ?

Also, i'm a little confused with if and only if and those little phrases sometimes confuse me.

[keg411]

Yes, yes, yes! Helps so much with inferences and speed as typically one question can be answered quickly with the help of a quick glance at the contrapositive!

BTW, figuring out contrapositives made in/out games click for me. I was so uncomfortable with them for a very, very long time.

What was it that made it click? I'm still a little iffy with contrapositives.

Are you supposed to take the contrapositive for every if statement? or only ones that start with a negative, like /R -> T so contrapositive that /T -> R. or if its X -> /Y should I also contrapositive Y -> /X ?

Also, i'm a little confused with if and only if and those little phrases sometimes confuse me.

I don't do every single one. The ones I focus on are like this:

Say the rule was:
"If Rachel is in the photo, then Tammy is in the photo"

I write:
If R ----> T
I would then write:
If no T ----> no R

For what you are talking about, let's say the rule is
"If Rachel is not in the photo, then Tammy is in the photo"
I write:
If no R ----> T
The contrapositive would be:
If no T ----> R
You also have to remember that this rule does NOT eliminate "R & T" from both being in the group!

Many of the times the answers in grouping (or in/out games) will come from having the contrapositive written down. Either that or it will make you see deductions much easier. I admit I don't always write them, but I'm decent at working backwards in my head. The best source that will help you with contrapositives is the LG Bible!!!!

[tomwatts]

Sometimes, on a clue of the form, "If G is before H, then both J and K are after L," I might skip diagramming the contrapositive (because the contrapositive is too complicated). Otherwise I pretty much always do.

[tikiman6]

Maybe on more complex games with thinks like, if G is included then neither L nor H are, or if G is included that either L or G but not both. These contrapositives can get dicey.

But in general, no. Simple if A then B, it's easy to remember that ~A when ~B, waste of time writing it down.

[dynomite]

For what you are talking about, let's say the rule is
"If Rachel is not in the photo, then Tammy is in the photo"
I write:
If no R ----> T
The contrapositive would be:
If no T ----> R
You also have to remember that this rule does NOT eliminate "R & T" from both being in the group!

See, this is a great example of why contrapositives are essential to LSAT success. Notice something about R and T in that example?

Well, if either one is out than the other is in... therefore... at least one of R and T must be in. So now right on my master diagram I fill that in. Say there are 4 total spaces in the group:

R/T _ _ _

[keg411]

For what you are talking about, let's say the rule is
"If Rachel is not in the photo, then Tammy is in the photo"
I write:
If no R ----> T
The contrapositive would be:
If no T ----> R
You also have to remember that this rule does NOT eliminate "R & T" from both being in the group!

See, this is a great example of why contrapositives are essential to LSAT success. Notice something about R and T in that example?

Well, if either one is out than the other is in... therefore... at least one of R and T must be in. So now right on my master diagram I fill that in. Say there are 4 total spaces in the group:

R/T _ _ _

You have to be careful on that, though... because R and T can BOTH be in using my example.