Stats 101, Part 2: Goalie Stats

You may have noticed that last week’s stats intro left out something important:  goalie stats.  Well, as you can imagine, the stats collected for goaltenders are quite a bit different from those collected for skaters.  Here are some examples, from potential Calder Trophy candidate Corey Crawford’s NHL.com page:

Basic single-game goalie stats

DEC:  Decision (Win or Loss)
GA:  Goals Against (shootout and empty net goals don’t count)
SA:  Shots Against
SV:  Saves
SV%:  Save Percentage (Saves divided by Shots Against, higher is better), but for some reason not actually expressed as a percentage
SO:  Shutout — in the single-game view, it will only be 1 or 0 (but if two goalies combine for a shutout, neither gets credited)
PIM:  Penalty Minutes (it can happen!)
TOI:  Time On ice (notice the overtimes)
EV GA:  Even-strength goals against
PP GA:  Power-play goals against
EN GA:  Empty Net goals against

Something’s apparently a little funny in NHL.com’s stats pages, since the “EN GA” column appears twice on each goalie’s page (or at least the ones I quickly spot-checked)!  I think the first “EN GA” column is actually supposed to be “SH GA”, standing for short-handed goals against.

And, like for skaters, there are

Basic season stats

Many of the columns are the same as for single-game stats.  The new ones are

GPI:  Games Played
W:  Wins
L:  Losses
OT:  Overtime wins
GAA:  Goals Against Average, lower is better (more on this below)
MIN:  Minutes played

And, as for skaters, there’s a row for the goalie’s league rank in each category.  Keep in mind, there are far fewer goalies in the league than skaters; still not too shabby a season for Crawford, especially when you consider that the defense in front of him has sometimes been . . . well, a little porous.

So what’s this “goals against average” business?
The formula for calculating GAA is

(Goals Allowed * 60) / Minutes Played

excluding shootout and empty net goals.  It is the average number of goals a goalie allows per sixty minutes of ice time (“averaging in” extra time played in overtime, or games where the goalie is pulled).

The most fundamental difference between GAA and SV% is that GAA does not factor in the number of shots the goalie faced.  Therefore, it’s generally considered more of a team-wide stat.  After all, the fewer shots on goal allowed by the team as a whole, the fewer goals will be scored against your goalie.  Of course, that’s not a given, though.  A hot goalie will post a low GAA even though he’s seeing tons of shots.  (And a slumping goalie may have a high GAA even if he has a rock-solid D in front of him.)

Because of the assumptions built in to GAA, I usually only consider it very useful when combined with SV%.  For instance, if a goalie has a nice, low GAA (not much more than 2.00), but a not-so-good low SV% (under 0.900), the most likely explanation is that the defense doesn’t allow a lot of shots on goal, but a high percentage of shots attempted are allowed in.  Total number of goals scored by opposing teams remains low (GAA), but a proportionately low number of shots are saved (SV%).  An unrealistic example of this would be a five-game stretch in which only five goals total were scored against the goalie.  Assuming the goalie played 60 minutes per game (for a total of 300 minutes), this would be a steaming hot 1.00 GAA.  But, during these five games, the defense was crazy and only allowed 15 SOG per game (75 total over the five games).  That makes a 0.667 SV%.  Yuck.

The opposite could also be true.  In another unrealistic example, take a five-game stretch in which 20 goals were scored by opposing teams.  Again, the goalie played 60 minutes per game.  That’s a terrible GAA of 4.00.  But, this time, the rest of the team was doped up on cold medicine and allowed 54 SOG per game.  That’s 270 SOG over 5 games, and 250 total saves, for a SV% of 0.926.

When both SV% and GAA are pretty good (or pretty bad), it’s hard to infer things about anything but the goalie.  However, when they look quite a bit different, it might say something about the rest of the team as well.

Well, this has become quite an essay!  I think there are probably at least a few lessons to be learned from talking about GAA and SV%:

  • Stats have the potential to be misleading;
  • Sometimes you have to do a little digging come up with a coherent picture based on numbers; and
  • Don’t ask a stats wonk a question about stats if you want a short answer!
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