Earlier in the season, I did a post on which teams had the best OPS differentials. So I thought it might be interesting to see what has a higer correlation with second half (actually post all-star) winning pct: first half (actually pre all-star) winning pct or first half OPS differential? Using the data from ESPN, here are those correlations for the years 2000-2007. The first half pct is the first number and the 2nd is OPS differential.
Interesting that the correlations were much higher in 2002-4. Overall, it looks like first half pct does a slightly better job. The average correlation for the first half winning pct is 0.46375 and for first half OPS it is 0.46463. So a very slight edge for OPS.
I expected a bigger edge for OPS since it gives a good idea of a team's performance and pct can be more affected by luck in a short time span. Maybe it reflects how good the closer or bullpen is and that carries over from half to half.