Saturday, July 17, 2010

Late-inning defensive substitutions

Sometimes a manager will remove a poor fielder from the game when his team is winning, believing the defensive improvement is enough to offset the loss of this player's bat. This usually happens in the 9th inning. Here's a short discussion about it: Tango analysis. My plan was to consider the following:
  • win: a binary response for logistic regression indicating if the team held on to win the game,
  • I.def: an indicator of whether the player was substituted for,
  • dist: the number of spots in the order until this player is due to bat again. Presumably as this increases, the defensive substitution starts to look better and better,
  • lead: either one or two runs,
  • inn: inning - limited to top of the 9th and bottom of the 9th. I didn't want to include the 8th and get into dealing with players who weren't replaced in the 8th but then were in the 9th. The replacement usually happens in the 9th inning anyway,
  • player: identity of the original player, much like the pinch running analysis, I'm going to lump all the replacements together,
  • opp: the opposing team,
and to limit the analysis to the AL games to avoid the implications of double switches that happen only in the NL.

To lead into my (short) analysis, I want to continue the selection bias discussion from my previous post. In that situation, requiring all four pairwise combinations of win/loss with pinch runner/no pinch runner to exist for each player created a selection bias and led to an overestimate of the PR main effect. I don't think it affected the analysis very much because my interest was in estimating the sum of this main effect and the average player:PR interaction for 25 good hitters, and this sum should not be affected by a biased main effect - the sum for an individual player estimates his personal PR effect and this is not dependent on any bias in the estimate of the average.

In today's analysis I am looking at defensive replacement by teams in the lead, so they go on to win the game - with or without the replacement - a vast majority of the time. Hence, if I set up the model in the same way as I set up the pinch runner model, with win as the response in a logistic regression, this selection bias will be quite severe. The pairwise combination most often lacking is the loss/replacement, and so many players whose replacement only ever led to victory get deleted from the sample. Proceeding as if everything was normal leads to a hugely negative defensive replacement main effect. Again my interest would be in the sum of this effect and the interaction effects of good hitters, so this is not an insurmountable obstacle.

The obstacle seems to be the lack of repetitions. I was picturing managers replacing their good hitters in close games all the time, and then when extra innings roll around, being left without their good hitters. But as an exploratory analysis I looked at 25 years of data, limited to players hitting in the top five in the order (lower than that the replacement might well be as good as the guy he's replacing) who've been substituted for in the 9th inning with a one or two run lead at least one time (not imposing any condition on having lost at least one game, and so unable to estimate a player effect), and found that out of the 36844 cases remaining, only 629 were defensive replacements - this amounts to less than 2 substitutions per season per team. And of those 36844, only 1490 times did their spot come to bat again - 1428 times with them in it, and 62 times with their replacement in it. At least you'd think that the team wins more often when the player hasn't been replaced, right? 657 wins/1428 games (46%) batting for himself, and 30 wins/62 games (48%) with the replacement - a statistically insignificant difference. Unsurprisingly, when I fit the full logistic regression model described above (again I limited to the top five in the order and fit without player:I.def interaction), the I.def effect was not anywhere close to significant.

This brute force approach to try to get around the noisy data is not going to work here. It certainly looks like the defensive replacement effect, if any, is quite minimal anyway. I'm satisfied for now, but to answer the question properly, I'd have to be able to measure exactly how much a defensive replacement helps the defense, and I don't have the data to do that right now.

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