Friday, August 13, 2010

An Introduction to Elo Ratings

One of the features of this site that I hope to build out even more are a set of proprietary Elo Ratings for Major League Soccer teams. The most up to date ratings can always be found by clicking that tab up above here that says MLS Elo Ratings. You may be familiar with the idea from the national team Elo ratings at or if you were ever a giant nerd who got really into chess. If you want some more information on how I developed it you can read this paper, otherwise just check out the 5 most common questions I get on it.

1. So what's the deal?

The Elo Ratings are based on a chess ranking system centered around a base value of 1500. A team's ranking goes up or down after each game depending on the quality of their opponent, the game's result, and the goal differential.

Anything over 1600 is up there with the best teams in MLS history. Anything below 1400 is super awful.

2. What about home field advantage, season-to-season carryover, expansion teams?

Based on historic MLS results home field advantage is worth about 85 points on a 1500 point scale, so we just add 85 points to the home team right before running the calculations.

When a new season starts, a team's rating moves halfway back to the 1500 midpoint. So if the Columbus Crew end the 2010 season with a 1540 rating they will start 2011 with a 1520 rating.

Expansion teams come in at a 1500 rating. It's not the best method by any means since most of these teams tend to be pretty poor, but it starts to correct itself over the course of a season.

3. Is it accurate?

Yes and no. It takes around 15-20 games for the ratings to normalize to the new team strength, which means at least half the season. Even then, if one team makes a big improvement during the season, like bringing in Thierry Henry and Rafa Marquez during the summer transfer window, it will take longer for the ratings to normalize than it will for a fan to notice who is playing well.

4. But my team is so much better than you say!?

Actually, even though I admit it's not perfect, odds are you are wrong here. Go back and look at your team's schedule. Look at how many of your wins were at home, or how many are against bad teams, or the overall goal differential. Beating the patsies by a goal will not help your Elo Rating very much at all.

5. Do you do other leagues?

Kinda. I've done the legwork of putting together the ratings for Premier League teams, but I haven't watched the changes for a season yet, so I'm not sure how good they are right now. It took a lot of work to get the MLS Elo Ratings developed since I used every results from the league's first season in 1996 and I want to make sure the other leagues are equally good before I unleash them on the masses.

I do answer a couple questions a month from people looking to create their own ratings though, so maybe you'll see some more pop up from other people.

Anyway, I hope that gives you a good background on how these work. If you have any questions just send me an e-mail or post in the comments and I'll get back to you as soon as I can.

No comments:

Post a Comment