Saturday, February 20, 2010

The Relationship Between Outshooting and Outscoring over Time

Derek Zona from coppernblue had a great post last month that examined the relationship between outshooting and outscoring at even strength over time. Specifically, he looked at aggregated EV goal and shot ratios for each team over the last three seasons as a whole (2007-08, 2008-09 and 2009-10, I presume). He found that the teams with the best EV goal ratios during this period were overwhelmingly teams that outshot the opposition at EV.

I think that his point is an important one. While the relationship between outshooting and outscoring may not be apparent over brief periods, the teams that succeed at even strength over the long run are those that spend more time in the opposition's end than their own.

Whereas Derek examined the strength of this relationship over the last three seasons taken together, I thought it would be interesting to look at the how this relationship varies over the course of an individual season.

The above table shows the correlation between EV goal ratio and Corsi ratio at the team level over certain game segments. The first bar shows the correlation between these variables over games 1-100 (-0.09). The second bar shows the same for games 1-200. The last bar shows the correlation over the entire season (games 1-1230).

It's apparent that the correlation between EV goal ratio and Corsi ratio increases over the course of the regular season. The increase is more or less linear over the first 1000 games, at which point it reaches asymptote.

The increase is even more pronounced if one looks at the relationship in terms of overall variance (r^2), rather than as a correlation. While Corsi ratio only accounts for roughly 9-15% of the variance in EV goal ratio over the first several hundred games, the r^2 value for the entire sample is in the range of 35-40%.

I've also included a chart that shows the same data for the 2008-09 season. While the increase isn't as sharp as that observed in 2007-08, the overall message is the same: as the season moves forward, the relationship between outshooting and outscoring at EV grows stronger.

Putting the Phoenix Turnaround into Context

I've written before about how much better Phoenix's EV numbers are this year as compared to last season. The Coyotes actually had the worst Corsi ratio in the league in 2007-08 at 0.83. They're currently sitting at 1.09, which is a pretty marked improvement considering that team Corsi ratios tend to be relatively invariant across seasons.

I was interested to see how this improvement ranks among team season-pairings in the post-lockout era. Looking at data from 2003-04 to 2008-09, I compared each team's EV shot data in a given season to the same data from the previous season. For seasons 2007-08 and 2008-09, I looked at each team's Corsi ratio. For 2003-04, 2005-06 and 2006-07, I looked at each team's EV Shot ratio. (While there exists data on blocked and missed shots from this period, it was much easier to simply scrape the shot numbers). Thus, it is important to keep in mind that for the 2006-07/2007-08 season-pairings, two different metrics are being compared (EV shot ratio for 2006-07, and Corsi ratio for 2007-08).

During this period, there were 120 season-pairings in total. For each pairing, I subtracted each team's Corsi/EV shot ratio in one season from that same team's Corsi/EV shot ratio in the following season, and ranked the differences in descending order. Presented below is a table showing the ten largest differences.

Turnarounds similar in magnitude to that exhibited by this year's Coyotes are a relative rarity. In the post-lockout period, only the 2007-08 Capitals and 2006-07 Leafs have shown a larger improvement in terms of EV shot metrics than the 2009-10 Coyotes.

The 2007-08 Capitals showed the largest swing. Although much of the credit for Washington's improvement has been attributed to the coaching change, the data suggests that the praise for Boudreau is misplaced. While it's true that the team struggled in terms of results early on, they were an outshooting team from opening day forward.

For the record, the coaching replacement occurred between games 20 and 21.

The Capitals weren't the only team to display a large improvement at EV in 2007-08. The Bruins also increased their Corsi ratio by a sizable margin. It's worth mentioning, however, that the difference in underlying play is probably overstated -- there's evidence that the Boston shot recorder is biased against the Bruins in terms of recording shots on goal, but impartial when it comes to recording shots directed at the net. (Keep in mind that the data for 2006-07 strictly includes EV shots on goal, whereas for 2007-08 all shots directed at the net at EV are included).

Even so, the Bruins were a much better team at EV in 2007-08 than in 2006-07. They also appear to have improved as the season progressed. As is typical of Julien-coached teams, they looked to have played to the score to a strong degree. (Note that the Bruins outscored the opposition at EV in the first half of the year, and were themselves outscored in the second half).

The 2006-07 and 2005-06 Leafs present an interesting contrast. While the two teams were fairly similar in terms of roster composition, the 2006-07 team was a substantially better team at EV. I tend to attribute this to the coaching change from Quinn to Maurice -- whereas Maurice's teams have historically been pretty good at outshooting the opposition at EV, Quinn's teams have not.

Of course, the Leafs' improvement at EV was nearly entirely offset by poorer special teams performance, which explains why the two teams finished at about the same place in the standings.

Finally, the Panthers brief post lockout improvement warrants some attention. The 2005-06 Panthers were actually a relatively decent team at EV -- they marginally outshot the opposition and fared even better in terms of goal differential (158 GF, 143 GA), largely on account of their goaltending. However, they were absolutely murdered on special teams (-35), and they missed the playoffs as a result.

The 2006-07 Panthers were even better at outshooting at EV -- their EV shot ratio of 1.17 was 3rd best in the league that year, behind only Detroit and Toronto. However, their special teams were again quite poor and, when combined with below average goaltending, they missed the playoffs yet again.

Interestingly, the Panthers would revert to their pre-lockout ways in the two subsequent seasons. Both the 2007-08 and 2008-08 Panthers were decidedly below average with respect to their EV Corsi ratio (0.94 and 0.87, respectively). I'm not quite sure as to the cause of the dropoff, although it's probably something worth investigating in the future.

EDIT: All statistical figures quoted do not include empty net goals.