Shot type bias: Rexall and wrist shots are like oil and water

A couple of days ago, I posted the shocking numbers about how few tipped and deflected goals the Oilers scored last season – dead last in the NHL by a wide margin.

That the Oilers were bad at this didn’t surprise anybody – but the degree to which they were bad was very much a surprise.

Extreme results like these naturally raise a lot of questions, and one of those questions was with respect to rink bias.

Rink bias is an acknowledged issue with much of the data that is recorded by the NHL, in that the fragile and subjective human beings recording this stuff don’t always do it the same way.  Laughed at by time, tricked by circumstances.

When the recorders in one rink consistently record a statistic differently than in other rinks, we call this rink bias.  Rink bias shows up very clearly in datasets like shots on net, shot locations, and hits.

Is There A Tip/Deflect Rink Bias at Rexall?

To determine if there is a rink bias operating at Rexall that artificially punishes the Oilers as far as their tip/deflect counts go, what I did is pull all shot attempts (i.e. Corsi) for 2014 and separated them into “Rexall” and “Not Rexall”.

I calculated the percentage of the total for each shot type for the two data sets, then calculated the difference in percentage.  If there is a rink bias operating at Rexall, you’d expect to see a significant difference between the two.

For the Oilers specifically, 4.4% of their shots were classified as tip/deflect at Rexall, while 4.1% were classified as tip/deflect when away from Rexall.

For all teams across the NHL, 5.6% of shots were classified as tip/deflects at Rexall, and 5.8% of all shots were classified as tip/deflects away from Rexall.

That’s basically no difference.

You can see that clearly when you look at a chart of those recording differences across all shot types:

shot type rexall vs not rexall

Interesting to see that the shot counters at Rexall are quite a lot less likely to call a shot a wrist shot vs a snap shot (relative to the rest of the league).  Definitely some rink bias on that front!

So if you’re ever looking at wrist or snap shots, you’ll definitely want to take that into account and assess with caution.

But otherwise, the answer is definitively no, it does not appear that rink bias played a part in the Oilers awful tip/deflect numbers.

The team gets to take full credit!


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