https://www.wired.com/2011/01/qa-scorecasting/

Bias of refs:

So we looked at how games are called and that’s where the data went berserk. Yellow and red cards in soccer, calls in the NFL before and after replay’s implementation, called balls and strikes in baseball — that’s where we saw games are called totally differently based on where they’re played. And the more attended the games are, the more striking the bias.