Trevor Cahill and Confirmation Bias

Trevor Cahill was supposed to become a premier starting pitcher. Oakland’s second-round pick in the ’06 draft punched out around ten batters per nine innings in the minors, compelled opponents to chop the ball into the dirt when they managed to make contact, and was dubbed the 11th-best prospect in the game by Baseball America prior to 2009. Last year, Cahill sliced his ERA from 4.63 during his rookie season to a svelte 2.97.

Entering 2011, fantasy owners have anointed Cahill as an elite option — MockDraftCentral shows he’s being drafted just slightly after the likes of Josh Johnson, Tommy Hanson, and Francisco Liriano. While Cahill undoubtedly made progress last year, the gap between his perceived acedom and actual performance might leave those shelling out a top pick with a sour taste in their mouths.

Cahill did make across-the-board improvements from 2009 to 2010. While not exactly hearkening back to those high-K days on the farm, Cahill’s strikeout per plate appearance rate climbed from 11.6% to 15.1% (18.2% MLB average). He also issued fewer free passes, paring his BB/PA total from 9.3% to 8.1% (8.7% MLB average).

The right-hander’s ground ball rate, 47.8% during his rookie season, shot up to 56%. That was fifth-highest among qualified starting pitchers, trailing just Tim Hudson, Justin Masterson, Derek Lowe and Jake Westbrook. Like those guys, Cahill’s sinker was the root cause: he got a grounder 62.1% of the time he threw the pitch, up from a MLB average 52% in 2009.

With more K’s, fewer walks and bushels of grounders, Cahill’s Expected Fielding Independent Pitching (xFIP) fell from 4.92 to 4.11. Much better, but nowhere near his actual sub-three ERA. The 1.14 run divide between Cahill’s xFIP and ERA was fourth-largest among qualified starters:

Cahill was able to outperform his peripherals due in large part to a .236 batting average on balls in play, lowest among all qualified starters. He had .153 BABIP on ground balls, compared to a .231 American League average and a .205 BABIP for all A’s pitchers.

It’s true, the 23-year-old has some built-in advantages that other pitcher’s don’t. The Coliseum suppresses offense, and the Athletics were a superb defensive team last year. Per Ultimate Zone Rating, The A’s saved about 39 runs more than an average squad in 2010. Josh Willingham aside, Oakland will again feature slick D in 2011. But even accounting for those factors doesn’t wipe away Cahill’s ERA/xFIP split.

According to Baseball-Reference’s Play Index Tool (details here), Cahill would have surrendered nine more earned runs last year if he had pitched in a neutral offensive environment in the AL instead of The Coliseum. As mentioned above, the Athletics’ D saved about 39 runs according to UZR. If we assume that Cahill got a defensive boost proportionate to the number of innings he pitched out of Oakland’s total (196.2 out of 1,431.2), then Cahill’s teammates saved him a little more than five runs.

So, Cahill’s ballpark and defense saved him around 14 runs total. In a neutral park behind an average defensive team, his ERA would have been about 3.60. Put another way, the Coliseum and Oakland’s defense explain about six-tenths of a run of the 1.14 run difference between Cahill’s ERA and xFIP. However, that still leaves a half-run gap that looks to be the product of good fortune on balls in play, above and beyond what could be expected even with a pitcher’s park and rangy fielders.

For 2011, PECOTA and Oliver project an ERA in the 3.60-3.70 range, while ZiPS is less bullish:

PECOTA: 190 IP, 6.1 K/9, 3.6 BB/9, 0.9 HR/9, 3.57 ERA
Oliver: 185 IP, 5.7 K/9, 3.3 BB/9, 1 HR/9, 3.68 ERA
ZiPS: 195 IP, 5.5 K/9, 3.1 BB/9, 1.1 HR/9, 4.07 ERA

Cahill’s in his early twenties, has upside, and benefits from both a home park that saps bats and a strong defense unit behind him. That being said, he’s not yet the ace that his 2010 ERA suggests. He could get to that level one day, but drafting him as though he’s already there is a recipe for disappointment.