The All-Star break isn't just a time to watch an exhibition and make tired This Time It Counts jokes. It's also a good time to reflect: on the months that have preceded, and where the players and teams will go in the second half. In a just world, a player with a certain level of performance would continue it after the break, and would nearly double their expected totals.
In reality, however, it's nearly twice as likely a player will decline significantly from their first half performance then they will continue or improve.
It would be best to think of what follows as a pilot test, as I only looked at the Diamondbacks. It also might be slightly limited in that I only considered batters with at least 150 at-bats in both halves with the Diamondbacks, and pitchers with at least 20 innings pitched in each of the halves. Still, it created a dataset of 214 players. Lastly, the metric used was fWAR, mainly because it was easier to compare the performances across position.
So Just How Likely Is It a Player Will Maintain Or Improve?
Most players don't see any noticeable change in the second half, and the average difference was a decline of .18 fWAR. Why even a slight decline? It could just be the effects of a long season grinding away on a player's endurance, but it's not really a great difference. It's essentially a player getting 2 fWAR in the first half, and then 1.82 in second. 3.82 isn't bad, depending on how much they're paid. It would be better than what any Diamondback did in 2012, if you need a little perspective.
Okay, so most players will see just a slight decline in the second half of the season. But how likely is it that a player might improve? Not so good, if we're considering significant improvement. Diamondbacks with an increase of .9 fWAR or more in the second half did so only 10.7% of time, and the increases were topped out with Steve Finley's 2001 campaign, where he did 2.5 fWAR better after the All-Star break.
Meanwhile, players that saw a decline of .9 fWAR or greater has happened 22.9% of the time. The biggest decline matches Finley, and funnily enough happened in the same season when Luis Gonzalez slipped back 2.5 fWAR.
Beyond the frequency, the real difference between the players that improved upon their first half, and those that did not, is also their starting position. Players that declined earned 2.47 fWAR on average in the first half, while those that improved only earned .19. This makes sense, where players that have both a bad first half and stick around for the second likely have no where but up to go. Players that have incredibly hot starts are probably due to slip back, unless you're a Randy Johnson type.
Okay, Nerd, What Does That Mean for the 2013 Diamondabacks?
Not so good, though perhaps not a disaster. Paul Goldschmidt, Gerardo Parra, and Patrick Corbin are likely all due to have a little cooler second halves. On the other hand, we might see an improvement in Jason Kubel, Martin Prado, and the bullpen disasters of JJ Putz, David Hernandez, and Heath Bell. but I wouldn't bet on it. They likely can't be much worse, but improvement is less likely to happen than a decline.
What that means for the team of the Diamondbacks is less clear. We still need Goldschmidt to rake, especially with the rest of the offense so hit and miss, but as noted above the biggest improvements and declines both happened in the same season, one that you might remember because Arizona celebrated its first, and still only, major professional sports championship.
That 2001 team, however, had more room to fall. Gonzalez had an absolute gonzo first half, and Finley started out ice-cold. Meanwhile they had two Hall of Fame caliber pitchers in their prime putting up big numbers on both sides of the All-Star break.
The probability might not be on some individuals sides, but luckily baseball is both a team game and game played between teams. Arizona is up 2 1/2 games heading into the second half, and that's proverbial Money In The Bank. Even better is the thought that if the Diamondbacks are likely to see slight declines from its best players, then other teams should likely expect that as well. The race ain't over yet.