Wednesday, July 23, 2008

The Effect of Playoff Seeding

I ran across an interesting stat recently. Patrick Roy opened the playoffs on home ice in 14 out of his 17 playoff seasons. Dominik Hasek opened the playoffs on home ice in 4 out of his 10 playoff seasons. Given those different situations, should we be surprised that Roy had more playoff success? How much does playoff seeding contribute to the success of a goaltender?

I decided to investigate the impact of playoff seeding a little further. The period I looked at was 1996-2008. This was so that variance from era effects wouldn't be too large, and also I wanted to look at seedings done by conference rather than by division to avoid strength of division as a factor. I took the starting goalie for each team (if there were several goalies sharing minutes, I took whoever played most in the playoffs as the #1 guy), and looked at both regular season and playoff statistics broken down by playoff seeding. This also allows us to see the team effect on goaltending - how do goalies tend to do on top teams as compared to weaker ones in terms of GAA and save percentage?

Looking at the aggregate regular season stats, GAA was very strongly correlated with seeding (0.88), as better teams usually give up fewer shots and fewer goals against. Save percentage was a much weaker correlation (-0.57). Higher seeded teams tended to have slightly better save percentages, but there was just a .006 difference between the highest-performing rank and the lowest one, compared to a 0.29 variance in GAA. This supports what I have been saying that GAA is very team dependent, and although save percentage does depend on the team to some degree it is much more dependent on the individual goaltender.

I also looked at the difference between the playoff stats and the regular season stats. GAA showed a clear correlation, as the top three seeds had average playoff GAA decreases of 0.18, 0.11 and 0.14 respectively, while number 8 seeds were hammered to the tune of a 0.34 increase in GAA. A similar effect was evident for save percentage - it went up .004, .004 and .005 for the top 3 teams, and down .004 for the worst team in each conference. The rest were around average, except for the 7th seeds which were a clear outlier - 0.15 GAA decrease, .010 increase in save percentage. There have been quite a few upsets in the 2 vs. 7 matchups which helps explain this a bit, but probably a major reason for this was J.S. Giguere's outstanding performance in 2003.

This illustrates a problem with the analysis - Giguere played 1,407 minutes at 1.62/.945 in 2003, but someone who did poorly and lost in the first round would not count for so much. For example, Steve Shields was the goalie for the #7 seeded team in the Eastern Conference the same season. Shields split playing time as his team lost in the first round, and as a result only recorded 119 playoff minutes at 3.03/.897. This means that Giguere's outstanding but unique playoff year was weighted 12 times as heavily as Shields' mediocre yet more typical one.

To correct for this error, I took the averages of the yearly GAAs and save percentages for each goalie, and compared them, thus weighting every season exactly the same. These results were very strongly correlated with playoff seeding. Average playoff GAA had a correlation coefficient with seeding of 0.93, which is very high. Average save percentage was also strongly correlated with seeding at -0.82, much higher than the regular season results. The #1 seeded goalies averaged 2.11/.913, while the #8 seeds averaged 3.03/.900. Here is the comparison of average regular season vs. playoff stats:

#1 seeds: 2.18/.914 vs 2.11/.913
#2 seeds: 2.34/.915 vs 2.34/.913
#3 seeds: 2.41/.911 vs 2.49/.911
#4 seeds: 2.34/.914 vs 2.48/.913
#5 seeds: 2.48/.908 vs 2.72/.904
#6 seeds: 2.44/.913 vs 2.70/.909
#7 seeds: 2.47/.911 vs 2.64/.908
#8 seeds: 2.50/.910 vs 3.03/.900

Another very strong correlation was between playoff seeding and playoff shots against per game (0.92). The top seeds saw just 24.4 shots against per game, while the bottom seeds faced 30.7.

A major reason for the discrepancy in the playoff numbers is that top seeds get to play against lower seeded teams in the earlier rounds. Facing weaker shooters and fewer shots per games gives those goalies a great chance to succeed. On the flip side, goalies on lower seeds are usually peppered by shots from top-level shooters. As a result, playoff goalies have large discrepancies in shot quality against. Take, for example, Hockey Numbers' shot quality stats from the 2006-07 playoffs, available on this site. Shot quality factors varied from a low of 0.66 (Dallas) to a high of 1.49 (Nashville). This range was over three times as wide as it was over the 82-game regular season, where the values were all between 0.86 and 1.11.

Looking at the regular season numbers, there is little difference in save percentage between teams at the different rankings, but GAA climbs steadily upwards as the teams get worse. This implies that goalie skill is fairly evenly distributed across the teams, and that a team's ranking is mostly determined by the skaters. I think it is reasonable that top teams would have slightly better goaltending on average, since they tend to prefer veterans and experienced players while weaker teams are probably more likely to take chances with castoffs or young players, but the numbers indicate this effect is not very large.

In conclusion, the effect of seeding on playoff performance is very strong at the team level. Seeding is very highly correlated with GAA, save percentage, and shots against per game in the playoffs. To a lesser extent, these effects are also visible in the regular season results. In both the season and playoffs, the difference between the best and worst teams in GAA is much greater than the difference in save percentage, which indicates that the primary cause of superior goal prevention is fewer shots allowed, rather than better goaltending.

This means that is mostly unfair to fault goalies who played on weaker teams for their lack of postseason team accomplishments. On the other hand, goalies who were routinely on a team that entered the playoffs as a high seed were in a much better climate for postseason success. Playoff team success, therefore, should be pretty much disregarded, and team strength and opposition should be taken into account when comparing playoff performance between goalies. For the most part, evidence suggests that goalie play actually has very little to do with playoff success. The Cup is almost always won by a top team, and the large majority of individual playoff games are won by the team with the better scoring chances. A goalie can occasionally have a large impact on a single game, but it is very difficult to be the single deciding factor in a series and virtually impossible to drag a weak team to a Stanley Cup.


a witness said...

once again...certain to draw critques on methodology. Can bias ever truly be removed - even from numbers!? However, i think you have good stuff wrapped in there...but i am biased anyway. Goalies get WAY too much credit.

Anyway, thanks for the time to do this...good stuff.

Anonymous said...

But what about Brodeur's puckhandling skills?

The Contrarian Goaltender said...

once again...certain to draw critques on methodology

I would welcome critiques on methodology. I presented both aggregates and averages, and I'm not sure how else you could look at this issue. I'm pretty confident that no matter what the method, the results will support the premise that in general, the better the team, the better the goalie stats in the playoffs.