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The NHL Has Fixed the Incorrect Shot Locations

Stats are back on the menu, folks

This is a follow-up update to the previous piece here.

Approximately three weeks after the issue was initially discovered, the shot location data in the NHL’s play-by-play listings have been corrected.  Shortly after the discovery was made by Josh and Luke Younggren of Evolving-Hockey.com, the NHL issued a statement that the transition to new analytics tools this season were likely the source of the issue, as also reported in this edition of 31 Thoughts by Elliotte Friedman.

Last night, the Younggrens announced that the majority of the first 91 games had been fixed:

Early this afternoon, Micah Blake McCurdy confirmed that the last remaining games were corrected as well.

(Note: I love Dr. McCurdy’s use of the phrase “discovered by outsiders”—it really gives the whole thing a post-apocalyptic vibe, which is fitting, since for us stats types it might as well have been the end of the world.)

While most statistics such as shot attempts, goals, etc. were unaffected, these issues had major effects on metrics such as Expected Goals (the probability of an unblocked shot becoming a goal due to factors such as location) and High-Danger Chances (another metric of goal probability, sorting shots into categories based on location).  I have updated the graphic from my last piece to show the difference these fixes have on these metrics.

Keep in mind that the sample size—about 5-7 games per team—is small, and the overall effect may not be immediately apparent.  However, for expected goals, one can usually expect a pretty close correlation between expected and actual goals scored over the course of a season, as seen in the top row of the graphic.

Interestingly, even with the fixes, nearly all models seem to show expected goals lagging behind actual goals this season—as noted by Brad Timmins, who runs Natural Stat Trick—and as we continue through the season it will be interesting to discover if this trend holds.  The difference between the incorrect data (0.53 goals over expected on average) and the now corrected data (0.29 goals over expected on average) is still significant.

Be sure to follow along here at On The Forecheck as I’ll be reviewing the analytics behind the Predators (and the rest of the NHL) now that enough games have been played and the data is accurate.

Because, even though the people who discovered this issue are actual professionals, this tweet does expose my state of being when I wrote the previous article:

I’ll have you know it’s my own house, and we don’t have a basement, thank you.

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