# Why is the PDO hockey statistic said to measure luck?

In hockey, the PDO is considered to measure luck. Could you please explain me why ?

From my understanding, the PDO is computed as "goals for / shoots on post for + saves against / shoots on post against". That is: your team is good at shooting (the guys score often) and you have a good goalie (he saves a lot). I don't see any luck here (that's also written in https://jfresh.substack.com/p/percentage-luck-in-hockey-explained : [..] there are elements of PDO that aren’t caused by luck, like shooting talent and goaltending).

When you sum the PDO of the two teams, you obtain 200%. That is: the two teams will share theses 200%. As the teams plays in the same league, there are more or less equally good, so it is expected each team get about 100%. We can find Gaussian distributions centered on 100% on the web (e.g. https://medium.com/hockey-stats/advanced-hockey-stats-101-pdo-part-3-of-4-d3f319f2e1f1 ). A wide Gaussian could indicate big differences between teams in the league, while narrow Gaussian could indicate tight differences between teams in the league. When low performing teams play against high performing teams, they learn and over years, they become better, making the Gaussian to be narrower (explaining why the PDO traditionally regress to the 100% mean, e.g. in https://jfresh.substack.com/p/percentage-luck-in-hockey-explained ). But again, I don't see any luck here.

Is "puck luck" an kind of "default reason" to explain phenomenons that we are not able to explain (like all the natural events that were considered as divine manifestations in ancient times, but are now scientifically explained) ? From my point of view, the defenders are under considered in the PDO (and in other metrics as well): they hinder the attackers so that the attackers shots are less dangerous, thus indirectly the defenders protect the goalie. The goalie save rate is higher because the shots are easier to stop, but the defenders impact is not measured (I agree it would be difficult to measure the "inconvenience" produced by the defenders). Also note that the opponent defenders have impact on our attacker goal ratio, so that the defender role has impact on the two components of the PDO.

I'm sure luck exists in hockey (we could get good bounces or bad bounces), but definitively, I don't understand how the PDO reflect it.