I hope you enjoyed part 1 of the Frequently Asked Questions at Behind the Net.  I'd like to discuss the second-most common question I get about the site: what exactly is Quality of Competition, or QualComp?

The basic idea is this: we want to characterize the characteristics of the opponents that a particular player lines up against. 

There are numerous ways we could go about this.  We could average out the points-per-game of opposing players(as Jonathan Willis has suggested, and which works reasonably well, particularly when you have no ice time information), but I think the best place to start is with what I called "Relative +/-" or "Rating" in part 1.  Relative +/- adjusts a player's on-ice +/- relative to his team's +/- while he was off the ice.  In general, it corrects for the boost players get from playing on a good offensive team and vice-versa. 

If we average that rating across all of a player's opponents, weighting for how much time they played against one another, then we have an estimate of how good a player's opponents were relative to their teams.  In a general sense, first line players have the best ratings, so players who play against the first line should see the highest opponent rating.  That average opponent rating is the "Quality of Competition" faced by a given player.

Here's a worked example from 2006-07:

If you lined up against Anaheim's top line, you'd get:

The Quality of Competition would be the average of 1.97, 1.65, 1.61, 0.94 and -0.31, which is 5.86/5 = +1.17.

So now that we have the preliminaries out of the way, what does Quality of Competition tell us?  Take a look at the 2007-08 list: Quality of Competition for 2007-08.  Niedermayer, Pahlsson and Moen are 1/2/3, reflecting their usage as the increasingly rare "checking line" in Anaheim.  Jay Pandolfo is 4th among forward, Mikko Koivu 5th, and John Madden 6th.  In 2006-07, Madden and Pandolfo were #1 and #2.  Nick Lidstrom's always in the top 10 too.  Basically, Quality of Competition tells us if players were used overwhelmingly against top competition, and it gives us another data point we can use to adjust a player's very high (or very low) +/-.

One of my favorite uses for it is to look at a team I don't know a whole lot about, and see what it says about defensive pairings and usage.  For example, the 2008-09 LA Kings:

Five guys, all within one minute of each other in terms of 5v5 ice time.  On many teams, it's obvious who the #1 and #2 D are because they log much more ice time than other players - both against other teams' good players and against not-so-good ones.  But if we add Quality of Competition to the mix:

Now it makes more sense.  The #1 pairing was Doughty-O'Donnell, though if Johnson hadn't been injured, he might have had more time on the first unit.  Matt Greene is clearly the #4 D, and Kyle Quincey is #5.  And this is generally consistent with how they were used last season.  On most teams, this method works out very well, though it's reflective of how the coach sees the player, and not necessarily of what the player's abilities are.