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.