Question on Page 221 of PS LGB (may 2009 ed) Grouping setup Forum

[e10]

Can someone please help me understand this better, I am confusing myself...

It says :

1. B <---/--->D

2. B--->G

3. C--->D

4. G--->B

And then says you can combine 1 & 3 to C<--/-->B

And 1& 4 to G<--/-->D

And then an inference of C<--/--->G can be made. I am not following any of this. If someone can please explain.

[Cambridge LSAT]

The inferences are a bit more obvious when you simply chain the rules together like so:
C ---> D <---|---> B <---> G

You could also diagram it without the double-not arrow:
G <---> B ---> ~D ---> ~C

Flipping and negating to form the contrapositive chain gives:
C ---> D ---> ~B <---> ~G

[esq]

Almost thought that you meant you were on the 221st page of your Lesbian Gay Bisexual Personal Statement, my bad

[Anaconda]

Almost thought that you meant you were on the 221st page of your Lesbian Gay Bisexual Personal Statement, my bad

Those jokes are seriously unoriginal.

[e10]

But why do you get to cut out the D & B to get to the C<--/--->G??

Does anyone have any strategies that work with grouping games? I have always struggled with them. I took a testmasters course last year, and messed up on the grouping game in sept and then in december. Also, maybe i'm wrong, but if i remember correctly testmasters would make you show whats in and whats out, but powerscore doesn't do that? is there a reason?

[pkrtbx]

But why do you get to cut out the D & B to get to the C<--/--->G??

Does anyone have any strategies that work with grouping games? I have always struggled with them. I took a testmasters course last year, and messed up on the grouping game in sept and then in december. Also, maybe i'm wrong, but if i remember correctly testmasters would make you show whats in and whats out, but powerscore doesn't do that? is there a reason?

Because if C is selected D must be selected, which means that B cannot be selected, so G cannot be selected. Instead of having to write that out repeatedly, it is a quicker way of recognizing that C and G cannot be selected together.

Same thing with G<--|-->D. If G is selected, then B has to be selected, and if B is selected then D cannot be.

[fosterp]

I personally hate the PS system of using bidirectional arrows and bidirectional not arrows. It seems rather than writing out a nice neat long chain inference the guide suggests making a hundred of these single rules for each variable that can affect another.

Personally I would look at it and combine them to get:

G -> B -> /D -> /C

And then you take the contrapositive of above statement:

C -> D -> /B -> /G -> /B

And basically there you have two rules that contain all four rules and their contrapositives. Whenever you are given a variable then you can pick the rule that has the variable and then follow it to the end, as long as you know you cannot go backwards on either chain. You can look at and easily see what can and cannot be possible much quicker than evaluating 6 different rules.

The B is in the second rule twice to demonstrate that you can go "backwards" from not g because of rule 2.