Thoughts on PDO and Luck
One of my beefs in the analysis and evaluation of hockey players is the notion that PDO (on-ice shooting percentage plus on-ice save percentage) can be used as a proxy for luck. A perfect example of how PDO is used as a proxy for luck is this article by Neil Greenberg about the Washington Capitals.
For example, when Alex Ovechkin has been on the ice during even strength this season, the team has a shooting percentage of 8.2 percent and has saved shots at a rate of .917. So that makes his PDO value 999 (.082+.917=.999), which is almost exactly the league average. In other words, Ovechkin has seen neither very good nor very bad “puck luck” this season.
What’s useful about this metric is that it’s “unstable,” and over a large-enough sample will regress to 1000. Why 1000? Because every shot that is a goal is a shot not saved, and vice versa.
My beef with such an analysis is the notion that for all players PDO regresses to 1000 and any players with PDO above 1000 are lucky and any players with a PDO below 1000 are unlucky. While I do believe luck can influence PDO over small sample sizes, not all players have a natural PDO level of 1000 and there are two reasons why.
1. Not all players play in front of perfectly average goalies which will have a major impact on the save percentage portion of PDO.
2. Players can drive shooting percentages.
To show you what I mean on point 2, I took 4 years (2007-08 to 2010-11) of 5v5 zone start adjusted data and grouped forwards based on their ice time over those 4 years and then calculated the on-ice shooting and save percentages and PDO for each group. Here is what I found.
PDO varies from 983.5 up to 1012.4 depending on the group’s ice time. This is largely driven by shooting percentage which varies from 7.5% to 10.4% with the players with the lowest amount of ice time having the lowest on-ice shooting percentage and the players with the most ice time having the highest shooting percentage. Order is the enemy of luck so seeing shooting percentages ordered this nicely tells me something other than luck is happening. Driving on-ice shooting percentage is a skill. This means more talented players can have a natural PDO (the PDO that they should regress to) above 1000 and less talented players can have a nautral PDO below 1000. Factor in the goaltending and a player could have a natural PDO well above or well below 1000.
Now, this is not to say that luck isn’t a factor in a players PDO, especially over small sample sizes, it’s just we can’t estimate that luck by assuming every players natural “regress to” PDO is 1000. Daniel Sedin has a PDO of 1043 this season (through Thursday February 2nd). Is it fair to suggest he has been luck and should see his PDO regress to 1000? When you consider his4-year PDO is 1035 (and his 3 year PDO is 1054) probably not. His natural, “regress to” PDO is probably not that far off his current 1043 PDO. Now if you are talking about Todd Bertuzzi this season it’s a different story. Through Thursday he had a a PDO of 1056 while his 4-year PDO is 994 and he hasn’t had a PDO above 1000 in any of the previous 3 seasons. It is probably fair to presume that Bertuzzi’s natural regress to PDO is much closer to 1000, maybe even below 1000 in which case it is fair to conclude that Bertuzzi has probably been quite lucky so far this season and is unlikely to continue at this pace the remainder of the season.
When used properly PDO can be an indication of luck but to do so we need to consider the context of a players PDO, not just assume all players PDO’s will necessarily regress to 1000.