Thursday, January 22, 2009

Mid-season coaching changes are correlated with disappointing records

The new issue of JQAS is out. Its second article, by three Swedish researchers, is called "Coach Succession and Team Performance: The Impact of Ability and Timing – Swedish Ice Hockey Data." The idea is that you can use a regression to try to figure out whether a coaching change helps or hurts the team. Alas, I think the authors are confusing cause and effect.

Their regression predicts this year's winning percentage based on a bunch of factors: last year's winning percentage, the coach's career winning record, the number of games coached, whether there was a coaching change, and whether that coaching change came in mid-season.

They find that mid-season coaching changes are associated with bad records, and conclude that it takes time for the team to adapt to the new coach. But isn't it more likely that the bad record led to the coaching change, rather than the other way around? Teams don't fire their coach when he's having a good season.

Actually, it's possible that the current season's record is only for the new coach; but, still, you'd expect a bad record in those cases. The previous coach was fired when the team underperformed, and (even considering regression to the mean) the underperformance is partly intrinsic to the team's talent. So the performance should still be similarly sub-par with the new coach, even if the coaching change didn't make any difference at all.

Another result finds that within teams, there's a high correlation between the coach's previous record and the current record – for all six teams studied, it was between .72 and .77. I didn't really expect it to be that high. You could assume that teams go through stages of badness and goodness, and each is likely to be associated with one particular coach. But I didn't think that would bring the correlation up as high as it did. Then again, there are only 21 seasons in the study for each team.

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At Thursday, January 22, 2009 8:42:00 PM, Anonymous Anonymous said...

Hey Phil,

Have you seen my article on the subject?

- David


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