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Jody Shelley vs Ilya Kovalchuk - Who's the better shooter?

Let's go back to a familiar chart - Jody Shelley's observed shooting talent vs that of Ilya Kovalchuk:

Shelley_kovalchuk1_medium

Just a reminder: shooting talent is the amount by which a player exceeds the expected shooting percentage based on the locations he shoots from.  I've only included initial shots at even-strength in this analysis - no rebounds, no power-plays.

Looking at that chart, it seems apparent that Jody Shelley could be a better shooter than Ilya Kovalchuk.  The probability is quite low - less than 1% - but Kovalchuk has more than a 5% chance of being a net negative shooter, while Shelley has a similar probability of being a net positive shooter. 

How is that possible?  Anybody who's ever watched Kovalchuk play knows he's a slick offensive player who can pick a corner as well as anyone in the league, while Jody Shelley is the definition of 'hands of stone' - there might be several million people in the world who can shoot better than he can.  Those observations aren't wrong - they just aren't perfectly reflected in the limited number of shots taken by the two players.

So how do we resolve this?  First, consider every shooter who 200+ shots over the last five seasons and look at the mean and standard deviation of their shooting talent.  Next, we simulate a group of league-average shooters over the same number of shots.  This allows us to determine the talent and luck involved in shooting:

 

Avg Std Pct
Total 6.36 1.53
Luck 6.36 1.03 45
Talent 6.36 1.13 55

 

So we can now start with an underlying distribution for each player - with a standard deviation of 1.13% - and update it based on how the player did in the five post-lockout seasons.  We get the following distributions for Shelley's and Kovalchuk's talent:

Shelley_kovalchuk2_medium

Now we have a much different story.  When we assume an underlying shooting talent distribution, we get an entirely different story - in this case, the likelihood that Jody Shelley is a better shooter than Kovalchuk is about 1-in-10000.  And it's probably lower than that - I binned my results in a way that doesn't really give us precision beyond the fourth decimal place.  Regardless, it's clear that Kovalchuk must be a better shooter than Jody Shelley.

Here we can see Kovalchuk's observed performance versus his likely talent:

Shelley_kovalchuk3_medium

Kovalchuk's true talent level - as implied by five seasons worth of data - is approximately 10.2%, which is 3.4 standard deviations above the mean of our initial distribution.  There's a small amount of uncertainty on that z-score, so it's possible Kovalchuk is just (just!!) three standard deviations above the mean.  Either way, Kovalchuk is no worse than the second-most talented shooter in the league, and most likely the most-talented.  And there's no question that he's better than Jody Shelley.

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This is a relevant place to ask this question, I think, and I’m pretty sure you’ve done work on this elsewhere, but how does shooting percentage ‘work’? Just by good old binomial probability, Kovalchuk was in the 5th percentile of shooting as a Devil. However, I’m pretty sure shooting percentage does not conform to that structure, nor should we exactly expect it to. I recall an article about how much of shooting percentage was talent and how much was itinerant chance.

I guess what I’m wondering is what were the odds that Kovalchuk would score 10 or fewer goals on 111 shots, given his career numbers.

by Triumph44 on Sep 9, 2010 11:00 AM EDT reply actions  

I don’t think that anyone else is doing this sort of sports analysis on any blog in any sport. Not with this level of execution. Props to you, Gabe.

Looking at your last chart, the green line is the joint probability of the blue line and the histogram of talent in the population (not shown, but you tell us that it is a normal distribution with a standard deviation of 1.13).

If that estimate of talent in the population is off by just a hair, say the variance is a touch larger and it is skewed a whisker to the right. Changes so subtle that it’s barely perceptable to the eye, what happens then?

Without executing that, I’d guess that Kovy’s histogram looks very similar, as does Shelley’s. But the ‘chances of Jody being a better shooter than Ilya’ goes from 10,000:1 to several billion to one. In short, it’s probably best to avoid talking about the edges of these probabalistic forecasts, or so I think.

That aside, fantastic stuff.

by Vic Ferrari on Sep 9, 2010 1:39 PM EDT reply actions  

Thanks. Your point about outliers is well taken. I have a bit of a numerical stability issue the way I formulated the problem – I really need smaller bins and a smoothed ability curve. Even then, as you point out, I wouldn’t want to make any claims about rare events.

by Hawerchuk on Sep 9, 2010 8:32 PM EDT up reply actions  

Cool stuff Gabe.

Earlier in the year you had asserted that there was no way to separate out finishing ability from practically any two NHL players not named Kovalchuk or Shelley.

At the time I thought that was a bit wild, I mean I’m not one to put stock in the “gotta bury your chances” narratives but I thought that (for example) a guy like Cammalleri, with his unreal windup from just on that of the post-to-dot line, would be victimizing goalies more often than average.

Anyways, does assuming an underlying distribution for the population separate talent from more than just Kovalchuk and goon?

by R O on Sep 10, 2010 12:12 AM EDT reply actions  

Thanks.

Jody Shelley’s biggest problem is that he never gets open for a shot. His lack of shooting talent is somewhat secondary to this. I think this methodology separates the best shooters from the worst, but it’s not clear that a guy like Andrew Brunette who gets into great position but has an average shot would come out a statistically-significant difference away from Shelley.

I think Cammalleri came out average too, didn’t he? I just looked it up and he’s about +0.5 goals per 100. I think he makes his spots more than makes his shots.

by Hawerchuk on Sep 10, 2010 1:29 AM EDT up reply actions  

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