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The Crazy/Hot Scale was made popular by How I Met Your Mothers' Barney Stinson. For those of you not familiar with the show, it is a diagram for relating the craziness, c, of a given girl, to her hotness h. Ratings are based off the standard 1-10 scale (10 being the hottest/craziest and 1 being the least attractive/most sane). For example, one of the most recent girls I hooked up with would be about a (5,8) - so 5/10 crazy, 8/10 hot.

The Line of Equality

As Barney explains, "a girl is allowed to be crazy, as long as she is equally hot". Let's make h a function of c. So, as c increases, h(c) must also increase at an equal or greater rate. I will call this line, the line of equality.

h(c) = c, c = craziness of a given girl

For a girl to be considered hook-up material, her (c,h(c)) must lie above the line, and thus, h(c) ≥ c.

To illustrate, a girl who is a 7 on the crazy scale must be at least a 7 on the hotness scale as well. If she's a (7,8) - great! You're getting more bang for your buck (per se). However, if she's a (7,6) - get out of there. Quick.
Bill: Hey I saw you talking to Beth at the bar last night. Did you slam that?
Jeff: No way - After she started talking about her hair collection I realized she was a 10/9 on the Crazy-Hot Scale and blew.
Bill: Damn! It is hard to lose a 9 like that!