Stability of Team Statistics

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DSMok1
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Stability of Team Statistics

Post by DSMok1 » Tue Dec 18, 2018 5:25 pm

Kostya Medvedovsky did some excellent work analyzing the stability of NBA team stats. This would also apply to lineup-level stats.

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DSMok1
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Re: Stability of Team Statistics

Post by DSMok1 » Fri Dec 28, 2018 5:13 pm

A thought experiment:

How much time is required for a coach to know if a particular lineup is working well? I know Crow has mentioned often that lineups are used for such short stints, it is hard to know how well a lineup meshes.

Plus/minus stats on even a few hundred minutes are extremely unreliable, because the underlying statistics have a huge amount of variance. For instance, just binomial randomness on a sample size of 20 or 30 3 point attempts will completely swamp any "signal" from a particular lineup combination generating particularly good looks. And that random error will completely swamp the actual plus/minus point totals.

As an example, I looked at lineups in the 2017 season, and compared the first half of the season with the second half of the season. Even lineups with over 100 possessions in both halves of the season showed almost no correlation in performance between the first half of the season and the second half of the season. (The correlation was 0.17). Only team stylistic choices showed good correlation between the first half of the season and the second--things like pace, 3PA, and DRB.

However, these are all viewed through the lens of rough counting stats. A team would be looking at things like the quality of 3pt attempts, if the team is struggling to get into plays on the offensive end, etc. Those elements should be apparent to the trained eye very quickly, compared to the crude measure of counting stats.

So my question is...how long does it take to assess a lineup, based on these more qualitative, yet basic, criteria?
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Rd11490
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Re: Stability of Team Statistics

Post by Rd11490 » Fri Dec 28, 2018 8:54 pm

A team would be looking at things like the quality of 3pt attempts, if the team is struggling to get into plays on the offensive end, etc. Those elements should be apparent to the trained eye very quickly, compared to the crude measure of counting stats.
So I looked at this last year in terms of expected points per shot vs points per shot (expected points per shot is just what the team would expect to be shooting if every shooter shot their career average from the location of the shot).

The charts for game by game can be found here:
https://imgur.com/a/Ti9Z94R

I can't find the code right now, but i'm pretty sure I found a very high correlation (>.9) between a teams first half ePPS and second half ePPS, but a lower correlation on PPS (~.7). On the defensive end I think both were in the .6 range.

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