I added these graphs to the comments of my post last night, but I think they're worth discussing on their own.
The explanation for what each of these categories refer to can be find in the procedure section of the post, here.
I compiled the numbers for both the Phillies and the Cardinals, to show the stark difference and perhaps explain a hidden factor in the 2011 NLDS that may have gone unaccounted for. Both the ERA and RA/9 graphs have been provided, to account for the effects of how errors are scored.
These graphs show how the Phillies (the whole team, not just the starting position players), hit against starters. These aren't perfect, as mid-inning relief appearances can skew the numbers, but there's no easy way to correct for that. Be warned, these graphs are concerning, to say the least.
The Phillies decline is massive, to the point where they end up well below league average against any of the three categories of aces.
The Cardinals, on the other hand, improve against aces compared to the league, and by a pretty decent margin. Does this explain how that NLDS went, even when the Phillies came in with 102 wins and the Cardinals came in with 90? Perhaps. Let's take a look at the Phillies offense against the Cardinals starters and vice versa to see if these trends held.
First, let's break up each teams rotations into the different categories of aces:
ERA- Aces Phillies: Roy Halladay, Cliff Lee, Cole Hamels
FIP- Aces Phillies: Roy Halladay, Cliff Lee, Cole Hamels, Roy Oswalt
xFIP- Aces Phillies: Roy Halladay, Cliff Lee, Cole Hamels
And now, the Cardinals:
ERA- Aces Cardinals: none
FIP- Aces Cardinals: Chris Carpenter, Edwin Jackson, Jaime Garcia
xFIP- Aces Cardinals: Chris Carpenter, Jaime Garcia
Okay, let's start with the Phillies Offense vs. the Cardinals Pitching
The same trends hold. The Phillies do far better against all the Cardinals starters than they do against the aces, especially in RA/9.
Now for the reverse, here's the Cardinals' offense vs. the Phillies pitching. The Phillies starters didn't give up any unearned runs, but the RA/9 seasonal totals vary, so I kept both charts.
There were only two different sets of data, as the FIP- aces were all the Phillies starters, and the ERA- aces were the same as the xFIP- aces. And in this one, the trend doesn't continue, as the Cardinals did worse compared to average against the Phillies top trio (they fared very well against Oswalt). However, the Cardinals still performed above what the league had managed, by a decent degree, whereas the Phillies were well below league average against the Cardinals starters.
It's still a 5 game sample on each side, so randomness is really far too prevalent to draw any real conclusions, but it seems like the trends continued on into the playoffs.
Sometime later this week I'll attempt the same type of analysis for the 2010 season and see if the trends continued for the Phillies vs. Giants.