The goalies in my sample were Patrick Roy, Dominik Hasek, Ed Belfour, Curtis Joseph and Martin Brodeur, the same goalies I had included in a previous study of playoff performance, so I was hoping to revisit that analysis with some updated information as well as to test the effectiveness of some of my estimates. I will get to the individual numbers in another post shortly, but first I'd like to present the aggregates because they have value in addressing some fundamental hockey questions about how teams play to the score and the effect of strategy on goaltender play.
1st two period stats:
28 SF, 27 SA, 2.52 GF/60, 2.16 GA/60, 9.0% SH%, .920 SV%
All of the 5 goaltenders had a lot of wins in their careers, and this line shows why. Some of the goalies played on outshooting teams while others were outshot, and some of the teams were better at scoring than others, but overall the percentages worked out to an average edge of a 1% higher scoring rate, which accounts for 0.27 goals per game. The rest of goal differential comes from the slight advantage in outshooting.
3rd period stats:
These third period stats are for the entire third period that began with a specific score. Ideally we would like to count based on actual game score throughout, rather than just at the start of the period, but that requires play-by-play data. My "box score method" is much simpler, but achieves it through a loss of accuracy. However, there were relatively few lead changes in the third period, especially since most of the sample was during a low period for goalscoring league-wide. The strength of the observed effect, even in this limited sample, is strong enough that I feel confident that these results do approximate the real effect. I would certainly encourage someone with the programming chops to break it down in exact detail for some recent playoff seasons to see if my results are typical.
When leading by one goal after 2 periods:
21 SF, 29 SA, 2.36 GF/60, 2.03 GA/60, 11.8% SH%, .928 SV%
When up a goal late in the game, the teams drastically cut back on offence. Similarly, the trailing team started putting more shots on net. Interestingly, however, save percentage went up and so did shooting percentage, implying that the trailing team is taking more shots of lower quality, whereas the leading team is generating higher-than normal shot quality. Another possible explanation for this effect is that the leading goalies simply played better (by focusing more, bearing down more, etc.) in this important situation. However, I would imagine that the goalies who were down 1 would also be highly-focused, and yet their save percentages went down almost 3%. Whatever the explanation, this resulted in a huge percentage gap (almost 5%) in scoring rates, which made the leading team actually slightly more likely to outscore its opposition despite the one-sided shot differential.
I doubt that is generally the case (remember, we are only looking at strong goalies here, and any save percentage below .919 would result in the leading team getting outscored), however it certainly helps to have a great goalie when you are trying to hang on to a one goal lead late in the game.
When tied after 2 periods:
26 SF, 25 SA, 2.34 GF/60, 2.08 GA/60, 9.2% SH%, .916 SV%
When tied, the outshooting results are similar, with some evidence of teams playing more cautiously. Many of these periods started out tied but then had one of the teams take the lead, so could be quite different than what the picture would look like if we looked at just the time when the game was actually tied. We will, however, see that situation that when we look at the overtime results.
When trailing by one goal after 2 periods:
30 SF, 22 SA, 2.64 GF/60, 2.01 GA/60, 9.0% SH%, .916 SV%
When trailing, the teams started throwing pucks on net and taking greater risks offensively. The scoring rate was identical to the rate in the first two periods, although save percentage was slightly lower. We would need to look at a larger sample to see if the shot quality for the trailing teams is generally lower. I would suspect that it might be, but I am not sure. The evidence does appear to be there that the leading team will probably have higher than average shot quality for. The goalies' save percentage was .004 lower than during the first two periods combined, and, and earlier we saw how the shooting percentage went up almost 3% when the teams were up by one.
This shows again why these teams were winning a lot of games - their scoring rates were better than the opposition's in all three situations. Part of this is probably that the teams sampled were more likely than usual to tie the game up and therefore some of this time would have actually been played with the score tied. However, the team leading after two periods had a very good record (86% of the time the team that was leading after 2 periods ended up winning the hockey game).
28 SF, 28 SA, 2.03 GF/60, 2.14 GA/60, 6.7% SH%, .926 SV%, 14 W, 14 L
In overtime, save percentage rose and shooting percentage went way down. It was interesting that these goalies' teams were likely to get outscored in overtime. Even though all the teams had a good goalie in net they were just as likely to lose as to win once the game went into the fourth period. This suggests that overtime is quite random. Note: Patrick Roy was a major outlier in terms of overtime results through the course of his career, but most of his overtime outperformance came in Montreal pre-1994 and is not included here.
Given how shooting results are apparently strongly dictated by the scoreboard, shooting results for an entire game are not particularly useful. If a team dominates the first two periods and goes up a goal or two entering the third, they are likely to sit back while the other team takes a lot of long-range shots in the third. The shots might have been something like 25-15 through two periods, but then get reversed 13-3 in the third, and both teams end up with 28 on the night even though Team A was clearly the better team. I observed many games with that type of distribution when going through the box scores.
These effects were less strong in the first two periods, however, so we can look at how outshooting determined the results over those two frames. I calculated a "winning percentage" based on the score after 2 periods, giving 2 points for being ahead and 1 point for a tie game.
When outshooting the opposition, the teams had a .635 win percentage. When outshot by any amount they were just .481. Given that we are only looking at the first two-thirds of a hockey game here, that is a lot of evidence in favour of outshooting driving results. Even with a Hall of Famer in your net (Joseph is debatable, but let's just assume he is for the sake of a more punchy sentence), evidence suggests that your team is more likely to be trailing than leading if you get outshot over the first two periods of a playoff game. TV analysts take note: The best way to predict a playoff series is not to simply pick the team with the better goalie, but to pick the team more likely to have the edge in puck possession and shots. Certainly teams can have the percentages go their way and ride a combination of a hot goalie and goals against the run of play for a few games or even a series or two, but past history suggests that the the inevitable hand of regression nearly always shows up to end the fairy tale short of the final prize.