For Pitchers That Change Teams, What Affects Their BABIP More? Their Own BABIP From The Year Before Or The BABIP For The Rest Of Their New Team?

This is related to the previous post Year to year correlation of BABIP for pitchers that changed teams and those that did not.

I ran another regression for the guys who changed teams. The dependent variable was their BABIP in year 2 and the independent variables were their BABIP in year 1 and the BABIP for the rest of their new team in year 2 (so that each guy's BABIP was removed from the team total). Here is the equation

BABIP2 = 0.232*OtherBABIP2 + 0.123*BABIP1 + 0.189

OtherBABIP2 means the BABIP for the rest of their new team in year 2

The r-squared is .022. Standard error is .02004. T-values are

OtherBABIP2 1.18
BAbip1 1.25

So neither is significant. But .232/.123 is 1.88. So the effect from the BABIP from the rest of the team (the guy's new team) is almost double from his previous year's BABIP.

The correlation between BABIP2 and OtherBABIP2 is .086, almost as high as .094, correlation between BABIP2 and BABIP1 for this same group of guys who changed teams (all the guys who had 150+ IP in the years 2003-14).

If we suppose that fielders have no influence on BABIP, then it means that the contribution from "the rest of the team" is due solely to the other pitchers. Then why is the effect  from other pitchers so much stronger than the pitcher himself (or at least what he did the year before)?

That would not make any sense. So it seems like there would have to be a big role for the fielders. If you think it is a "small" role, but not zero, the previous paragraph still applies.

The previous post showed that for guys who stayed on the same team, the impact of last year's BABIP on this year's BABIP was 2.5 times higher than it was for guys who changed teams.

And what is the big change when you switch teams? The fielders. The parks, too. The pitchers have a whole new set of fielders behind them and they pitched half their games (or around that) in a completely different park than the year before.

I don't know how much the makeup of a team's fielders change from year to year. My guess is that it is less than 50%. So guys who stay on the same team don't see much change in fielders and most of the time no change in park.

So it looks like the big difference between guys who change teams and guys who don't is the fielding. You could say guys staying on the same team see a change in fielding. But that is probably small compared to the guys who change teams.

And you could say that for guys who change teams the change in parks is a big deal. But you only pitch about half the time in one park, whereas it probably the same mix of fielders behind you all the time. So again, that points to the fielders.