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Is a team with a PDO of 101.973 a lucky team?

Note: any data used here is based on the numbers available on the morning of 3/29/14 before any games were played that day.

Is it? According to Extra Skater only three NHL teams currently have a PDO higher than 101.973, the Anaheim Ducks (103.4), Boston Bruins (103.0) and Colorado Avalanche (102.2). We know that a team's PDO is the sum of the team shooting percentage and the team save percentage. Since every shot on goal results in either a save or a goal the sum of all league totals equals 100.

What do we know about PDO?

Here's how Behind The Net.ca defines PDO:

PDO is the sum of "On-Ice Shooting Percentage" and "On-Ice Save Percentage" while a player was on the ice. It regresses very heavily to the mean in the long-run: a team or player well above 100 has generally played in good luck and should expect to drop going forward and vice-versa.

Hockey conventional wisdom is that PDO is a measure of luck (or a proxy for a measure of luck) with a baseline of 100 (or 1.00 or 1,000.00 depending on which power of 10 you're using). According to this conventional wisdom a team with a PDO greater than 100 is the beneficiary of luck and can expect their PDO to trend downward to 100 as more games are played and their data set increases. So is a team with a PDO of 101.973 the beneficiary of luck who can expect their PDO to trend downward?

I've long maintained that this conventional wisdom is faulty and based on bad assumptions. PDO is named after the name of a commenter on an Edmonton Oiler's blog who coined it. Here is what he said that lead to the assumption that the baseline for teams/players should be 100:

Lets pretend there was a stat called “blind luck.” Said stat was simply adding SH% and SV% together. I know there’s a way to check what this number should generally be, but I hate math so lets just say 100% for shits and giggles.

That's where this conventional wisdom comes from, I'm not making this up. (Source)

A much better description of PDO comes from Cam Charron:

‘Regression’ here is the theory that since every shot taken in the NHL must result in a save or a shot, the mean PDO in the NHL is 1. The longer a player or team plays, the closer its PDO will get to 1.

You'll notice that he doesn't say that the after a team plays for a long time it's PDO will be 1 (or 10 or 100 or 1000 depending on what power of ten you're using), he says that the longer a team or player plays the closer it's/his PDO will get to 1.

A team or player's long term PDO equilibrium is not necessarily 1 (or 10 or 100 or 1000 depending on what power of ten you're using). It's critical to understand this if you're going to look at PDO and interpret the meaning of its measure for any team or player. This is something the team at Pensburgh captured well:

There is, however, one cautionary point: when dealing with individual PDO's, one needs to take the context of the player into account. If Eric Godard finishes with a 99% PDO and Sidney Crosby has a 101%, does that mean Godard will get only get better and Crosby will get worse? No, because the talent differential between these two players means that they have different average PDO's. Since Crosby is one of the best players in the world, he'll have an above average PDO. That should be the benchmark by which you judge him, not the general 100% average. Yet discovering each player's "true" average PDO is very difficult, and the sample sizes only become big enough once a player is about 30 years old.

So let's get back to the original question, is a team with a PDO of 101.973 a lucky team? Well let's go through an exercise for the purpose of this exercise we're going to assume that the team in question has a team shooting percentage of the league average, nothing special.

Let's look at a real team and use actual data, the current Boston Bruins (as of the morning of 3/29/14, through 73 games played). Looking up the even strength save percentages of their goalies this season, the combined totals for even strength goals/shots against for this season's Boston Bruins is that they've given up 103 even strength goals against on 1,717 even strength shots against for an even strength save percentage of .940. Adding up the even strength goals against and shot totals for every goaltender in the league this season gives you a league average even strength save percentage of .921, so the Boston Bruins are earning 1.9 PDO points over the league average on the goaltending side this season just for the saves their goaltenders are making.

Are their goaltenders lucky or good? Well I can't find any career totals for even strength save percentage but I can find career totals for all situations save percentage. Tuukka Rask has made 71% of the Bruins saves this year at even strength, his all situations save percentage this season is .931 which is just a shade above his career average of .928. Chad Johnson has made 27% of the Bruins saves this year at even strength, his all situations save percentage this season is .925 which is just a shade below his career average of .926. Are the Bruins benefitting from lucky, unsustainable goaltending? It doesn't appear that way, it looks like they have very talented goaltenders who are having season totals pretty consistent with their career totals. The Bruins goaltending numbers seem entirely sustainable and not due to luck.

Now let's say for argument's sake that this year's Bruins team had a league average shooting percentage. Well the league average even strength shooting percentage is 7.9%. (1 - league average even strength save percentage of .921 = .079) Now remember that the Bruins don't have to shoot against their own goalies, so if you take out the Bruins save totals from the league totals you get a league-wide non-Bruins even strength save percentage of .920 so an average shooting Bruins team would have an expected even strength shooting percentage of 8%.

Now if you combined the Bruins actual even strength save percentage with a hypothetical league average shooting ability and took out some of the rounding you'd get a PDO for this team of 101.973. We've controlled for any luck on the shooting percentage side and we've shown that the save percentage pretty well tracks with career totals.

A team with a PDO of 101.973 isn't necessarily a lucky team benefitting from unsustainable play. Depending on the skill of the players involved it could be due to luck or it could also be due to actual ability. PDO may be a proxy for luck but it isn't a very good one. It doesn't isolate only those factors that can be specifically attributed to luck. PDO is the plus/minus of advanced hockey stats, it's only a matter of time until we have a much better metric of the luck it is attempting to measure.

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