Imagine you're a Phillies fan who had a blast watching the team's early season success. Life's life, so somewhere around early May, you got really tired and put your head down to rest. Through the magic of imagination, what you thought was going to be a short nap turned into a month-long snooze.
When you went to sleep in the middle of the day on May 4, your beloved Phillies were the shock of the majors. They had a 16-11 record, boasted an exciting starting pitching staff that was striking out everyone, and were bucking all the off-season tank criticism. But now, when you woke up in the middle of the day on June 3, your team is below .500, its starting pitching staff is erratic and often mediocre, and the offense has seemingly completely disappeared.
What happened? Where have the early season Phillies gone? To figure this out, let's break the season so far in half - the first 27 games (coincidentally for our record-setting nap, ending on May 3) and the second 27 games (May 4 through yesterday).
Let's start with the team's record and run differential.
On first glance, it's obvious - the team is scoring fewer runs and allowing more. However, the differences aren't drastic. Scoring 5 fewer runs and allowing 7 more runs should not translate to a 6 game swing.
Maybe our favorite Greek mathematician can explain what's going on. It was widely noted in the Phillies early run that they were out-performing their Pythagorean expected win percentage, and that was clear. Have they reverted to the mean in the second 27 games?
The Phillies are no doubt performing more in line with their Pythagorean expectations, but they are still out-performing, just not by as much as they did early in the season. In other words, they're still getting lucky, just not nearly as lucky as they were early on. (Of course, the flipside of this luck is that there's still room for the team to get much worse in the coming weeks and months.)
Is it strength of schedule? As much as we have the Cubs and Nationals fresh in our minds from the recent streak of losses, the Phillies have actually faced more weak teams in the last 27 games than they did in the first 27. In the first 27, 10 of their games were against teams with records under .500 (3 against Milwaukee and Cincinnati and 4 against the Reds). In their last 27, 13 of their games were against teams under .500 (another 3 against Cincinnati, 3 against Detroit, 6 against Atlanta, and 1 against Milwaukee).
How about looking at the hitters and pitchers in the two different halves? Here's the offense comparison in the first 27 games compared to the next 27 games:
The big difference here? What little power the Phillies had in the first 27 games has gone further downhill in the second. They are doubling less and homering less, resulting in a miniscule .117 isolated power over the last 27 games. They are also walking less, though their on-base percentage is virtually the same thanks to a slightly increased batting average.
For the Phillies' pitchers, it's a different issue - their batting average on balls in play has gone up. Here's the comparison:
Although the pitchers are allowing a higher on-base percentage and slugging percentage, the entire difference can be explained by the increased batting average allowed. Proof - the isolated slugging of the opposing teams is pretty much exactly the same.
Two big differences though are evident - the pitchers are allowing a higher batting average on balls in play and they're striking out fewer. That's a bad combination - fewer strikeouts (with home runs and walks remaining pretty much constant) means more balls in play; with a higher batting average for balls in play, that means more hits for the opposing team (and thus a higher overall batting average, leading to a higher OBP and SLG).
So where does the blame lie for the change in the pitching from the first half to the second? Your instinct that it's the starters is correct:
Here we see the stark difference from the first 27 games to the second 27 games. The relief pitchers have produced much better results in the second half, but the starting pitching has been much worse. They're still not walking many batters, but they're striking out less, allowing more base runners, and giving up much more power. That's a bad combination.
All in all, the difference from the 16-11 Phillies of the first 27 games to the 10-17 Phillies of the second 27 games can't be captured in one single thing, but rather a bunch of little things: they're scoring less, allowing more runs, getting less lucky, hitting for less power, giving up more base hits on balls in play, striking out fewer batters, and starting games off in a hole.
Put it all together and we have a six game win difference from one half of the season so far to the other.