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08-13-2012, 02:51 AM
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Some in this thread are claiming that PDO is a primarily luck-driven statistic.

Others are claiming that some teams will have significantly better PDO numbers than others over the long run.

These statements are not necessarily incompatible. Both can be true, depending on the context.

With respect to the first statement, it is true that, at the level of a single NHL regular season, most of the variation between teams is luck.

This can be measured with some precision - if we look at each season from 2003-04 to 2010-11, for example, we find that 61% of the team variation in PDO was due to luck, and 39% due to skill.

This carries implications for estimating a team's true talent level with respect to PDO. For example, during the above noted period, if we single out the teams with the best PDO number in the league in each of the seven seasons, they posted an average mark of 1.025. Because most of the league-wide variation is due to luck, however, our best estimate is that a team with a 1.025 PDO number actually has a true talent PDO number of only 1.00975.

This corresponds well with the observed data - the teams in the above study posted an average PDO number of 1.0083 in the following season.

Of course, merely because most of seasonal variation in PDO is due to the luck isn't to say that all teams have the same underlying talent with respect to PDO. The skill standard deviation at the team level is about 0.0074.

So if the team with the best underlying PDO talent in the league is two standard deviations above the mean, it would have a true talent PDO number of 1.0148. That's a sizable advantage. The problem is that because most the variation over a single season is due to luck, that team's observed results will vary significantly around that mean.

The Bruins had an average PDO number of 1.011 in each season from 2007-08 to 2010-11, but the range over that period was considerable (0.991 - 1.029).

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