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10-30-2013, 02:26 PM
  #36
hatterson
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Quote:
Originally Posted by Freudian View Post
Since the save percentage for clean shots is so stable, wouldn't the fact that we have such differing save percentages for goalies over a season suggest that all goalies doesn't face the same ratio of clean shots?
I'll address Taco's response below, but just a couple notes here:

On ratio of clean/dirty shots:
We know (based on trivial observation) that all goalies don't face the same ratio of easy and hard shots in a single game, so it's at least possible that they don't on a season level. Either way, the real question when we're looking at projecting isn't whether goalies *did* face more/less clean shots, it's whether teams have a control over this in the future.

The question of whether they did or not is of interest if we're comparing past seasons, or perhaps comparing different goalies for awards in a given season (IE, is a .930 with 95% clean shots more impressive than a .925 with 85% clean shots?), but it doesn't tell us much about the future. Just because Bernier faced a high number of clean shots in one season doesn't mean he will in the next, *UNLESS* we can show that a team has real control over shot quality against. That's the real question that needs to be addressed for this to have predictive power.

On the differing save percentage:
The article states the best-worst difference is 38 goals over 1,000 shots or .038 save percentage. Even assuming that all goalies face an identical ration of 85% clean shots that .038 difference can still result in .0323 save percentage difference over a season. That's the difference between .905 (guy everyone wants to run out of town) and .9373 (vezina favorite).

The variations we see in current goaltender performance seem to be well within the ranges even if shot quality turn out to not be a thing on the aggregate level.


Quote:
Originally Posted by Taco MacArthur View Post
Ah, that makes more sense - thanks!

If anyone's got better eyes than I do (even for a goalie, my eyesight's pretty bad) and wants to eyeball the range of "clean shot" save percentages, I'll defer to you.
Based on zooming in to the pixels it appears that each pixel is worth .00227(27) in save percentage. Unfortunately the dots are 4-6 pixels high so we can't get exact, but I took a decent guess.

It appears the numbers fluctuate from a low of .9409 to a high of .9795 so it appears your eyes are pretty good. Although that would mean the average was .954228 rather than the stated .951 in the article. However the difference between high and low is .0368 or 36.8 goals per 1000 shots, which is very close to the 36 stated in the article.

Either way, the numbers we can extrapolate from the graph seem close enough to reality to draw some conclusions.

First, I completely agree that a variation between .9409 and .9795 doesn't seem very stable. The goalie at .9795 is nearly three times as good as the goalie at .9409. 38 goals per 1,000 shots may not be a big difference if the range is 580-618 out of 1,000, but it's pretty significant when the range is 941-979 out of 1,000.

To extrapolate that difference into something a little more familiar lets assume 30 shots per game with 25 being clean, 2 being transition 2 being rebound and 1 being being deflection (That's a rough average of what 30 shots would turn out to based on the percentages in his article, it's not ideal, but it gets the point across)

Goalie A is at the top end with a .9795 save percentage on those and Goalie B is at the bottom end with a .9409. For all other goals the goalies are perfectly identical, each stopping an exactly average amount of shots. This means a .693 save percentage in transition, .760 on rebounds and .736 on deflections.

Now each goalie will let in an average of 2 * (1-.693) = .614 transition goals, an average of 2 * (1-.760) = .48 rebound goals and 1 * (1-.736) = .264 deflection goals for a total of 1.358 goals against per game.

Now for clean shots. Goalie A will allow an average of 25 * (1-.9795) = .5125 goals. Goalie B will allow an average of 25 * (1-.9409) = 1.4775 goals.

That would bring their total stats to

Goalie A: .938 save percentage, 1.8705 GAA
Goalie B: .905 save percentage, 2.8344 GAA

That's a *massive* difference which means I'd strongly disagree with the idea that the clean save percentage numbers aren't that big a deal.

Edit: And to further show the importance. Being at the low end vs. high end of either of the other 3 categories only makes the GAA move +/- .25 or so. Certainly not minuscule, but if you could choose between Best at clean worst at everything else, and worst at clean best at everything else, you're better off with the first option by a significant margin (roughly .924/2.28 vs .910/2.70)

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Last edited by hatterson: 10-30-2013 at 02:36 PM.
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