Wednesday, October 22, 2008

Save Percentage and Shots Against

Recently, at Hfboards and elsewhere, I've come across the assertion that, among NHL goaltenders, there is a positive relationship between save percentage and shots against. Some examples.

Naturally, I was skeptical of the claim
, not least because no compelling evidence was ever offered in support of it. Moreover, there doesn't appear to be any reason why this should be so. Conceivably, there could be some degree of trade off between shot quality and shots against at the team level, in that teams that are better at preventing shots do so at the expense of allowing more quality ones (at least proportionately) and vice-versa. However, there doesn't appear to be much of a correlation between the two.

The chart shows the correlation between shots against and shot quality against (at the team level) in the right column with the corresponding season in the left column. In only two of the seasons (2002-03 and 2003-04) was there something of a relationship. What's interesting, however, is that the correlations are positive, meaning that, in those particular seasons, the teams that were better at preventing shots also tended to allow lower quality shots. There is absolutely no evidence of a trade off between shots against and shot quality.

Of course, it's possible that there could be a relationship between save percentage and shots against independently of shot quality. It's often been suggested that goaltenders play better when they're frequently tested and poorer when underworked. If this were true, a correlation between save percentage and shots against should emerge at the team level. But what does the data say?

As can be seen, there isn't much of a pattern between the two variables. In some years, the correlation is positive; in other years, negative. What's important, I think, is that all the values are fairly close to zero. The only year where the correlation is even remotely significant at the 5% level is 1999-00, but the direction is contrary to what would be expected . The important point is that teams that give up more shots against do not tend to have higher save percentages.

There is absolutely no evidence that high shot totals have an inflationary effect on goaltender save percentage.

Why, then, is it often argued that such a relationship exist? More than anything else, the phenomenon seems to be driven by wishful thinking on the part of the claimants, a disproportionate number of whom belong to certain fan bases. I'll say no more.


Vic Ferrari said...


Good stuff. The impact of the team on shot-quality-against is quite small according to the hockey anaytics model (and surely much of that is a result of an unbalanced schedule and randomness). I mean sometimes even in one game they review a couple of goals and determine that both or neither) were deflected well after the fact.

The fact that it repeats so well season to season shows that it has veracity, at least to my mind.

And as you say, if you could look at odd numbered games vs even numbered games in the same season, the reliability would surely be much higher.

Of course there is a whack of randomness in save percentage, and I doubt that SQA would have much predictive value (though SQF probably would, just looking at the relatively higher magnitudes of impact).

In any case, is there anywhere that posts SQA's for PP shots? I suspect that is a much bigger factor, and is probably driving the overall numbers.

Preferably 5v4 PP shots data would be gold.

I couldn't find any breakdowns at hockey analytics.

JLikens said...

Thanks for the comment, Vic.

Your right in that it's unfortunate that neither Ken nor Alan decomposed the shot quality data by game situation (EV, PP, SH).

However, Chris at hockeynumbers also posts data on shot quality and does break it down situationally.

Here is the link:

One caveat: The shot quality data for each team is for all of that team's games, and not simply for road games. Thus, I'm not sure to what extent his data is biased by the home arena effect and whether or not he has taken any steps to correct for this.

Hostpph said...

I don't know why they don't like to have supportive material to prove that they are right. They just reach to a conclusion.