## Wednesday, January 21, 2009

### Scoring environment

Recently, Brian has made his win probability (WinEx) calculator available for all to use. This is a really powerful toy, and I plan on using it for some stuff I'll post about later. For now, however, I want to point out a flaw.

One of the first things one notices when looking at NFL stats is that they lack context. How valuable is a 3 yard run? Is it a three yard run on third and two? Is it from the 2 yard line, when down by 6 with 20 seconds left? Is it in the middle of the field, as time expires, down by 27? Obviously, context is important. It only gets worse as the stats get bigger. How valuable is a 1000 yard rushing season? Well, if that's 15 rushes with 13 touchdowns, it's the best season by anyone, ever (and the question is "why did he not get more touches?"). If it's 400 rushes with 2 TDs, the question is "why is he not on the practice squad?" It's all about context -- in this case, the context of the event: what were you trying to achieve?

This extends further, however. How good is throwing for 6000 yards in a season? Well, it'd set a lot of records. Unless, of course, defensive players are on strike and teams are running amateurs out there every weekend, causing 12 quarterbacks to throw for 6000 yards. How about scoring 200 points across the regular season? Kinda crap...unless you have the best defense of the last decade, and are 14-2 when the dust settles. Again, it's all about context -- this time about the context of the achievement: how did everyone else do?

WinEx calculations are an attempt to solve the first series of questions I posed. The second is a little trickier. How does the system know that being up by 12 with 5 minutes left is good? Because those teams usually win. Note, however, that this is "those teams," not that team. In this case, the league scoring context is being used to determine how often teams tend to score.

The question is, is there another context each game is played in? I wouldn't have written all this unless the answer is "yes," of course. There are actually a couple different contexts: team-level and game-level.

A team-level context is simply which teams are playing. Let's say the Steelers and Raiders are playing. Just before the opening kick-off, you're asked who's going to win. If you're looking at things in the "NFL" context, it's 50/50. But if you know who the teams are, it's certainly not. This becomes less and less true over the course of a game -- if the Raiders are up by 14 with 20 seconds left, they've probably won, whether or not they're the Raiders -- but before the opening kickoff, it is not "anybody's ballgame".

There's another context which is important for calculating WinEx, though, and that's game context. Let's say your team is up by 10 with 8 minutes remaining. That's a pretty good lead, right? Well, if it's a 13-3 game, it's a VERY good lead -- if their poor opponents couldn't muster more than a field goal in 52 minutes, they're unlikely to close a 10 point gap now. On the other hand, if it's a 42-32 shoot-out, it's still anybody's ballgame -- the hometown heros haven't stopped them yet, and they're unlikely to start now. One piece of good fortune on an onside kick, and you could be drinking away the evening, trying to forget how the WinEx calculator told you the game was in the bag.

I know that Brian's WinEx calculator doesn't take these things into account -- it doesn't ask what teams are playing, and it asks for score differential, not current scores. It may never take them into account, and that won't stop it from being a useful tool. But there's more to think about when analyzing in-game scenarios: that 3 yard rush is meaningless without context.

#### 1 comment:

Brian Burke said...

Follow up on this topic at PFR here.

Bottom line (to me) is that the total score doesn't matter nearly as much as score difference. I.e. 10-0 is almost the same as 30-20. But team strength is a big factor. A 10 point lead going into the 4th quarter against an inferior opponent is very different that a 10 point lead against a superior opponent.