Setup and Rule Diagrams

[pwfquestions]

Why can't you draw the FK rule as a not block?

[Adam Tyson]

You absolutely CAN do that, pwfquestions! In fact, that was how I chose to do it, rather than the double-not-arrow that we used in our explanation. The difference is one of style, not substance, and you can choose whichever tool you prefer.

[CristinaCP]

Could you also write the K -> J rule as K -> [KJ] block?

[Dana D]

Hey Cristina,

You cannot write K J as a [KJ] block, no.

[KJ] written as a rule would mean that whatever committee K is on, J must also be on, and vise versa. They must always be together.

However, that is not what K J says as a rule. K J says that whenever K is on a committee, J must also be there. J is the necessary condition and can occur on its own. If we are told J is on a committee, we cannot automatically assume K is as well.

This is especially important to remember with games like this one, where there is no rule that every volunteer is selected for a committee - what if J is on planting and K is not picked for anything? That does not violate the rule K J , but it would certainly violate the rule [KJ].

[CristinaCP]

Hi Dana,

Sorry, I didn't mean just writing [KJ] as a block. I see why that's wrong. I meant writing that when K is selected, K must occur with J, which would look like this: K --> [KJ]. Whenever we see K, we must also see K with J.

Can't we write it like that?

[Adam Tyson]

You could do that, CristinaCP. Much like the earlier question about a double-not-arrow vs. a not block for the FK rule, this is a matter of style, not substance. It's true that if you have a K in one of the committees, you must have a KJ block in that committee, and K (KJ) captures that nicely!