Originally posted on 03/12/2011:

Whatever is tough to account for is equally tough for the bettor and for the bookies. Some of the changes you mentioned have a little impact on the "global" stats anyway. On the other hand, a bettor may have an edge due to bookie not accounting for a certain factor F. There are two completely different takes on this. One bettor creates a model and makes a line for a given game, say, -5 while accounting for a factor F. The bookies' line is -3. This bettor knows that he has an advantage and keeps wining until the bookies catch up a few seasons later and start hanging -5 on a similar contest making our bettor realize that now he has no edge and there is no bet. Another bettor notices that betting the games with factor F produces a winning ATS record. He starts betting every game that meets that criteria. He does not know when to stop and will end up a loser due to the followers dilemma.