Thursday, February 2, 2012

tempo-free lacrosse

If you're a lacrosse fan, or a math fan, or a lacrosse fan who likes math, then here is your post.  This is my attempt at mathifying the game.  Inspired by Ken Pomeroy's well-known basketball ratings, and a similar tempo-free approach to the game of lacrosse espoused by Great Lax State (a blog about lacrosse in the great state of Michigan), I decided to make an attempt at breaking down college lacrosse in a Pomeroyesque fashion.  Let me give you the numbers first so you know what we're building to; the explanation follows.  These are from last season.

Lacrosse, like basketball, is at its heart a game of possessions and how well you make use of them.  For our purposes here, there is one major difference: in basketball, possessions alternate without exceptions.  For every possession by the good guys, there is always a corresponding possession for the bad guys, excepting that you win the opening tip and then have the ball at the end.  But it's still a 1-for-1 deal.

In lacrosse, it's 1-for-1 except that scoring and game periods result in a faceoff, not an automatic trade of possession.  In basketball, you can count possessions with stats from the boxscore; in lacrosse, I believe we can do the same.  A team can begin a possession one of three ways:

- Win a faceoff.
- Gain the ball on the defensive end.
- Gain the ball on the offensive end.

The boxscore gives us faceoff numbers, of course.  How do you gain the ball in your offensive end?  A successful ride - that is, a failed clear by the opponent.  Also in the boxscore.  How do you gain the ball in your defensive end?  Any number of ways, but they are counted in the boxscore as either clears or failed clears.  Thus we have three ways to mark a lacrosse possession, all of which are in a standard boxscore:

- A faceoff win.
- A clearing attempt.
- A failed clear by the opponent.

So a team's total possessions in a game can be determined by adding faceoff wins, clearing attempts, and failed clears by the opponent.  Note that we leave ground balls out of it, because ground balls tell us nothing about who lost it in the first place, or where.  If you win a ground ball in your defensive end and successfully clear it, that'll show up in the boxscore.  If you win a ground ball in your offensive end, but you lost the ball to begin with, then we don't count that as a change of possession.  Possession is lost only when the other team clears (or fails to, but they had the ball and the chance to) or at the next faceoff, whether that faceoff was the result of a goal or a new quarter.

Another important difference between lacrosse and basketball is that here, we're marking the beginning of a possession.  The way KenPom does it in basketball is to mark the end.  For lacrosse, this is a more accurate way to do it.

In totaling up last year's stats, it turns out that almost exactly one-third of possessions start on a faceoff.  Tangent: this is why I will, from here on out, bang the drum that faceoff percentage is overrated.  A typical game is about 70 possessions.  (Let's say 69 for divisibility purposes.)  This game would have 23 faceoffs.  If you win 56% - an excellent number - that's 13 of 23, and since all other possessions are one-for-one by definition (that is, after a faceoff, teams will trade clears until someone scores or the period ends) you get half of the remaining 46 and 13 of the faceoff 23, for 36.  The other team gets 33.  Your prowess at the faceoff X gave you just 52% of the possessions.  While that can swing the tide in a close game, it's not the end-all, be-all that it's often portrayed as.  People freak out about losing too many faceoffs, and it seems logical to do so, but much more important is your clearing game, offense, defense, etc.  Does it matter?  Absolutely it matters.  But only when faceoff percentage gets really large - over 60% or so - does it start to have a major, freakoutable impact.

OK, anyway.  Faceoffs are one-third of possessions, and the rest are one-for-one.  This is where that difference from basketball comes to get us, because we can't split the possessions evenly; we have to weight them.  The final numbers above show how many goals a team would score and give up in a 100-possession game; how do we get there from here?  If Team A is expected to score 16 goals in a 100-possession game, where do I get that number?

-- First, we determine how many of those 100 possessions belong to Team A.  That's easy.  33 possessions are allocated to faceoffs and 67 are split evenly between Team A and their opponent, Team B.  Team A gets 33.5 possessions plus their faceoff percentage times 33.  The equation:

33.5 + (FO% * 33)

-- Next, we need to know how many of those possessions made it to the offensive side of the field.  You can't score if you don't get into the box.  (OK, you can, but we're ignoring acts of God here.)  Faceoff wins are assumed to always make it to the offensive side, because the boxscores don't differentiate.

Last season's data shows that of the 67 non-faceoff possessions, 57 start on the defensive end and 10 are the result of a ride.  Therefore we give Team A five offensive zone possessions and add them to the possessions given them by faceoffs.  Then we look at their clearing percentage.  Multiply their clearing percentage by 28.5 (half of 57) and add the result to the above.  The resulting equation is:

(FO% * 33) + (CL% * 28.5) + 5

Now in our fictitious 100-possession game, we know how many times Team A had the ball in the offensive zone.  In real life, we know how many goals Team A scored (obviously) and we also know how many actual offensive possessions they had, because we can add up their clears, rides, and faceoff wins.  Simply dividing goals by offensive possessions gives you a percentage, which, multiplied by the offensive possessions per 100 we just came up with, gives you the team's final O-rating.

And you can repeat the whole process for defense as well.  Essentially the D-rating is each team's opponents' O-rating, as if the combined opponents' stats were for one team.

There's your explanation.  Let me now try and pre-emptively fend off a few questions:

Why does it say "raw" O-rating (and D-rating) on the header?

Because I don't have a good way of adjusting for strength of schedule.  Yet.  I do have one way, but it's crude and not fit for public consumption and not even useful til at least three-quarters of the season is over - although it does at least do a better job of putting the best teams at the top.

Why not simply rank the teams by goal percentage and goals-allowed percentage, instead of all that rigamarole about possessions?

That does have its appeal.  But it doesn't tell the whole story.  By rolling up faceoff percentage and clearing percentage into the statistic, you get a better sense of how dangerous a team really is.  Take Yale, a team that was very close to making the tournament last year.  They had a very pedestrian, middle-of-the-road goal percentage - 28th.  Their phenomenal faceoff percentage makes them much more dangerous than the Marists of the world, though, and they are 13th in this O-rating calculation.  Likewise, a very good defensive goals percentage plus that faceoff prowess makes them tough to score on.

What if a team is credited with a clear directly after winning a faceoff?  That would skew their possession numbers.

Smarter lax fans than I will have to speak up and say how often that happens, if at all.  I don't know.  If it doesn't, great; if it does, oh well, the boxscore doesn't make a distinction, and so I have to work with what I got.  I thought about that early on but realized, either way, I can't do anything about it.  So I stopped giving it any thought.

But if a team's riding ability is really good, shouldn't that give them an offensive advantage, and vice versa if it's poor?  Instead of just handing out the same number to everyone to "account" for the ride?

Kind of.  I would indeed like to refine this thing a little more to include that.  For the sake of accuracy and accountability and waterproofing the formula and all that.  But as a rough go, it's pretty close; successful rides aren't all that common to mess with the numbers too much.  And the truth is that unless a team really presses (which rarely happens outside of endgame situations when a team is trying to come back) whether or not the ball is cleared has a lot more to do with the clearing team.

Are you going to update this during the season?

No.  If you haven't noticed, I'm a damn superstar at promising I will continuously update something and then not doing it.  I will do it on my own during the season, use the numbers in previews and analysis and such, and share them at the end of the year, and upon request.

Can't you make this damn thing sortable?

No.  I just write.  I took one programming class in college and it was the biggest mistake I ever made in those four years.  Someone wanna help me out in that department, be my guest.

I have more questions!

Then ask them.  Nothing like a little scrutiny to help make these things better.

1 comment:

Anonymous said...

Awesome stuff man