Calling political races

What specific criterion do the news organizations use to decide when to call the race? Do they just call the race when they have a significant statistical sample of the votes? Do they rely on exit polls? Astrological charts? Do they cut open some livestock and check out their entrails?

Judging from the fiasco last night, not all of the networks use the same criterion. NBC seemed to be the most cautious about calling races, while CNN was bouncing all over the place. was blaming their reporting service for the flip flops today. Perhaps they don’t even have a coherent policy!

Personally, despite the new gray hairs that sprouted on my head while I was watching the results last night, I couldn’t be happier that the networks made fools out of themselves. We needed another Dewey Defeats Truman to deflate the importance of the polls in elections. Perhaps if the candidates didn’t think they could rely on the information coming in from their tracking polls they would be forced to actually take stands on issues instead of just giving the answers that seem to be polling the best at the moment. But that’s a pipe dream, right?

Anyways, what’s the straight dope, ma’ams and sirs? Who makes the call?