For quite a few years I have pondered the meaning of the percentages provided as official measure of power play and penalty killing success.  To me, they never seemed to be a fair assessment of a team's PP or PK ability because

1. A PP/PK of any length counts toward the total.  Team A, Player 1 takes a tripping call at 19:57 in the 3rd and it counts as a PK when it is inevitably killed off, helping Team B's PK ranking and hurting Team A's PP ranking.
2. If Team A scores two goals on a PP from a major penalty, are they 1 for 1 or 2 for 1.  They aren't 200% on the PP for that game, are they? They were successful on 100% of their PP's but showing them as 1 for 1 is also not really indicative of the game situation.
3. In last year's painful first period against Calgary, where Tyler Sloan earned the Caps a 9 minute power play,  Washington was able to do precisely nothing with it (well, they did manage to turn it into a 7 minute PP).  On the official score sheet, this counted as 0 for 3 on the PP (2 minors and 1 major), however the team managed to score no goals on 7 full minutes of PP time.

This last one really got me thinking about this issue and I occasionally dabbled with writing a script to parse the RTSS data and figure out how good a PP team the Caps really are.  I was even more motivated (yet still managed to do nothing) when the Caps finished the year ranked 2nd on the PP.  One part laziness + one part not knowing how to quantify things + 3 parts grad school forced this to the back burner.

A couple of weeks ago I realized that PP effectiveness might be measured just like the Goals Against Average, i.e., Goals (scored or allowed) per 60 minutes.  I've adapted my parser to handle the 2008-2009 and 2009-2010 seasons (2007-2008 will come, they changed the page formatting some), so we can look at the results.  Thanks to F&B for the suggestion to look at Goals per 2 min of PP and the number of PPs per Goal.  I've added these columns to the tables and rearranged things a little to make it more readable.

First, the 2008-2009 season, since that data is complete and we have a firm idea of where teams should be:

The important things to see in this chart are the 2nd and 3rd columns, which contain the PP effectiveness as measured by the league and my method.  The 8th and 9th columns have each teams rankings under the two metrics and the last column contains the difference.

So, what do we learn from this?  The answer is not much.  It appears that those rare arbitrary situations don't affect the final rankings too drastically.  But, as fans we like to see that the Caps moved into the #1 spot.  (Don't get your hopes up for the PK data, though.)  The Devil's drop of three spots is interesting though, I'm not sure what to draw from that. 

One thing that we learn is that the Capital's have really short power plays, on average, when compared to the rest of the league.  This is clearly due to good goal scoring but I'd be willing to wager some of this is due to taking bad penalties that end PPs early.

For completeness, here is this season's data (through Friday, October 23):

Interestingly, the Caps still have a far shorter average PP time than teams that are near them in the rankings.  I wish I could explain that somehow.  We'll have to see as the season goes on, I'll update my numbers and post them again sometime a month or two from now.

An interesting note from looking at the PP/G data is that the current method for computing PP efficiency seems to overestimate the number of PPs required to produce a goal by more than 1 on average.

Stay tuned for data on penalty kills.  That data is processed but I havn't had a chance to look at it in too much detail, that and I'm still figuring out what these data mea, if anything.  Additionally, if anyone is curious, I actually have the breakdowns for one- and two-man advantages as well as both 5-5 and 4-4 even strength times, so I can spit those into a table at any time.