Oh for god's sake, you don't need all these extra things to see that pitchers improve as they come to Wrigley... a hitter's park. It's that simple.
No, you don't. But you do need those things to say with any confidence whatsoever that Larry Rohschild was the cause of those better performances. Simply seeing a correlation and immediately attaching whatever cause first presents itself (though I admit the pitching coach is a logical choice, if not a bit brash) is a trap that many SABR-minded people fall into. Is there a correlation between pitching performance and Larry Rothschild? Yes, but absolutely no causation has been proven, only implied based on minimal-depth looks into the topic. And even after all those things are accounted for (change in venue, age, change in league, etc.), what remains to be demonstrated is that the change in pitching performance people want to attribute to Rothschild (here from the perspective of the individual pitchers) are out of the ordinary, that not all or not even a goodly amount of pitchers experience similar rises in performance after a "change in scenery".
Ummm, did you even bother to read that article outside of the big table? Specifically, this part?
Unfortunately, HR/PA did not become reliable at the samples that were studied (up to 750 plate appearances). Its correlation at 750 batters faced was .3. Although the samples won't reflect reliably an improvement or worsening in home run rate, we can still look at it just for fun.
Let me give you some idea as to how completely unreliable that is when applied to players with at most 750 PA's (or 12 members of the sample): from year to year (or however they broke it down, from start to start or batter to batter, whatever), the previous HR/PA mark set by that individual can only explain about 9% of the value of the next cycle's HR/PA. That is hardly the mark of a repeatable skill--at least at those sample sizes--and to extrapolate meaning from statistics garnered in such small sample sizes (many of which are far less than the 750 maximum) is utterly erroneous.
And oh yeah, where is the adjustment for the HR/PA rate of the pitchers before their time with the Cubs? Take Ted Lilly, for example, the numbers, as presented, would have you believe that Rothschild had Lilly giving up HR's at a lesser rate, but for the vast majority of his career up until that point, he had been playing his home games in the Rogers Centre, a stadium that inflates homers even more than Wrigley. So really, his slight decrease in HR rate is completely attributable to his change in venue, and not necessarily Rothschild's influence.
