Shots Attempts Against

With the issue of shots against featuring prominently in recent discussion here and elsewhere in the blogosphere, I thought to take a slightly different look at the issue. Generally attempts to define the shots against/save percentage relationship look at total numbers by season (like this one, for example). I was interested in how teams performed over multiple seasons, and how similar shot preventing teams did in terms of save percentage.

Based on the results from the even-strength save percentage while tied numbers, I was also interested in exploring the results of blocked shots and missed shots against. The frequency of blocked or missed shots is quite high. Over the last 8 seasons, there were 555,531 shots, 215,508 blocked shots, and 208,889 missed shots, meaning that 43% of shot attempts never make it on net. It would therefore seem that a team's commitment to shot blocking could have a big impact on the number of shots actually faced.

The "fewer shots = lower save percentage" argument is usually based on one of the following two premises: 1. Goalies who face infrequent shots lose focus and are less physically prepared for each shot that comes than those who are facing more frequent shots, or 2. Goalies who face infrequent shots are facing higher quality chances. The first one could possibly be true, but based on my personal experience I do not think it is a major factor. It is, however, a difficult one to test (it would require analysis of the play-by-play records to do it properly), so this post is directed at premise 2.

Before I continue, just a caveat: with RTSS data there is always the possibility of systematic errors. It is, in fact, quite likely that teams differ in their reporting criteria, based on what we have discovered from past shot and shot distance reporting results. I will present the data as is, but if there is reason to believe that the numbers aren't quite correct please point it out.

I took the last 8 seasons (1999-00 to 2007-08), and collected the total minutes, shots against, saves, blocked shots and missed shots for each team. I then divided the teams up into 5 groups, based on where they rank in shot attempts against (Att), defined as saves plus goals allowed plus blocked shots plus missed shots. I have also included shots against (SA) and save percentage numbers for each team.

Low Shot Group:
Anaheim: 46.4 Att, 28.2 SA, .913 Sv%
Detroit: 46.5 Att, 25.9 SA, .910 Sv%
Chicago: 46.7 Att, 28.2 SA, .899 Sv%
Dallas: 46.9 Att, 25.0 SA, .910 Sv%
San Jose: 47.1 Att, 27.0 SA, .910 Sv%
New Jersey: 47.5 Att, 25.5 SA, .912 Sv%

If a low shots against total was the only handicap preventing Martin Brodeur from posting elite save percentage numbers then we would expect the Devils' goaltending to outperform the rest of this group. The fact that Anaheim, Detroit, Dallas and San Jose have virtually identical save percentages while facing even fewer shot attempts than New Jersey seems to be strong evidence against that viewpoint.

Anaheim and Chicago do not block many shots, at least according to NHL scorers, so even with average shot totals they both move up into the top group here. Chicago is definitely the outlier, finishing far behind the other teams. Could the Chicago scorers possibly be underreporting blocks and missed shots? Or do the Hawks simply suffer from bad goaltending and/or high shot quality against?

Moderately Low Shot Group:
Boston: 48.4 Att, 28.5 SA, .903 Sv%
Nashville: 48.7 Att, 29.3 SA, .912 Sv%
Ottawa: 48.8 Att, 26.8 SA, .908 Sv%
Vancouver: 48.8 Att, 27.2 SA, .906 Sv%
St. Louis: 48.9 Att, 25.4 SA, .901 Sv%
Calgary: 48.9 Att, 27.5 SA, .907 Sv%

St. Louis ranks near the very best in shots against, but the reason seems to be that they block so many shots. Given the goaltending they have had that is maybe not too surprising, but lumping in St. Louis with the top possession teams during the study period appears to be incorrect. Ottawa is similar to the Blues. On the other hand, Nashville apparently lets a lot more shots through, and they had the best save percentage in this group.

Average Shot Group:
Colorado: 49.3 Att, 27.4 SA, .912 Sv%
Tampa Bay: 49.5 Att, 28.3 SA, .898 Sv%
Philadelphia: 49.8 Att, 27.1 SA, .908 Sv%
Toronto: 49.9 Att, 27.9 SA, .903 Sv%
Buffalo: 50.0 Att, 27.9 SA, .909 Sv%
Minnesota: 50.0 Att, 28.5 SA, .916 Sv%
Phoenix: 50.0 Att, 29.4 SA, .905 Sv%

The average group had the widest range of save percentage results, from Minnesota (best in the league) to Tampa Bay (worst in the league). Even though Minnesota blocked a normal number of shots and allowed an average number of shots on goal, their goalies had very high save percentages, which suggests that Jacques Lemaire knows how to make life easier for his goalies. Both Colorado and Buffalo had good results even after Roy and Hasek left town, which suggests that they were good defensive teams.

Moderately High Shot Group:
Columbus: 50.3 Att, 30.3 SA, .905 Sv%
Carolina: 50.4 Att, 27.5 SA, .901 Sv%
Los Angeles: 50.8 Att, 27.7 SA, .900 Sv%
Washington: 50.9 Att, 29.9 SA, .905 Sv%
Edmonton: 50.9 Att, 27.2 SA, .904 Sv%

The save percentages are very similar for all 5 teams and none of these teams had elite goalies in the period (other than maybe a couple of Kolzig seasons), so the results likely generally reflect team shot quality against. Edmonton had the second highest total of blocked and missed shots in the league while Columbus had one of the lowest, creating a difference of over 3 shots on goal per game, but the two teams had almost identical save percentages.

High Shot Group:
NY Islanders: 52.2 Att, 29.6 SA, .904 Sv%
Pittsburgh: 52.2 Att, 30.4 SA, .900 Sv%
Florida: 52.9 Att, 31.9 SA, .913 Sv%
NY Rangers: 52.9 Att, 29.1 SA, .904 Sv%
Montreal: 53.3 Att, 30.1 SA, .914 Sv%
Atlanta: 53.3 Att, 31.6 SA, .898 Sv%

For teams that give up a lot of chances, the norm appears to be mediocre save percentages rather than higher ones. Florida and Montreal are the only above-average teams and they rank far ahead of everyone else.

Here are the overall averages for each group:

Low Shots: 46.9 Att, 26.6 SA, .909 Sv%
Mod. Low: 48.8 Att, 27.5 SA, .906 Sv%
Average: 49.8 Att, 28.1 SA, .907 Sv%
Mod. High: 50.7 Att, 28.5 SA, .903 Sv%
High Shots: 52.8 Att, 30.5 SA, .906 Sv%

There is no real evidence of a pattern in terms of goaltending success. There are a few teams that have outlier results based on their groupings, either underperforming (Chicago, St. Louis, Tampa Bay) or overperforming (Nashville, Florida, Montreal, Minnesota) their expected save percentages. If we remove the best and the worst team in each group to deal with potential outliers, the save percentages by group go .911 - .906 - .907 - .903 - .905. This evidence certainly doesn't show that playing on a top defensive club hurts one's save percentage; if anything it suggests the opposite. However, save percentage is not a perfect proxy for shot quality - the goalies themselves obviously have an impact. If top teams tend to have better goaltending, for instance, then we would expect this result.

These numbers are polluted by a few variables, like suspect RTSS data and special teams play. The topic of possession, outshooting, and scoring percentages is still being investigated. However, I remain unconvinced that playing on a strong defensive team makes it tougher to put up high save percentages. I did not really evaluate possession effects here, merely shot prevention, so it could be that there is an effect for outshooting teams. In any event, even if a general relationship is established, this post is evidence that there is still substantial variability from team to team even within similar shots against groupings. This variability means that we cannot necessarily go from an established general result to the specific case for individual goaltenders or individual teams without additional supporting evidence.

These findings also show significant differences in shot-blocking tactics, which support the use of Fenwick or Corsi numbers rather than raw shot totals to evaluate team puck possession. Finally, there is some evidence of a relationship between blocked shots and goalie save efficiency (-0.30 correlation between blocked shots per game and save percentage). It remains to be determined whether this may be because shot blocking makes it harder to stop pucks, or because teams with bad goaltending simply try to make more saves themselves.