There are two areas of basic methodology to cover: Ratings and Predictions.
One of the things that makes sports so exciting is the debate that inevitably ensues after every week of games about which team or group of teams are better than the others. The human polls tend to weighed down by emotion and bias, whereas the computer polls can’t actually watch the games. Out goal was to make a set of ratings that followed common sense by trying to mimic human polls with a solid statistical element driving it.
Statistical theory is obviously very important, but experience has taught us that theory rarely translates to practicality. In general, the underlying basis of our ratings are built around logistic regression. If you would like to be bored with the details of logistic regression then please reference this Wikipedia article. This method is, in fact, not much different than a lot of ranking systems already in place. Our system differs in that it weighs each game differently based on a variety of factors that we humans use either consciously or subconsciously to rank teams.
When DRatings was first built, it was only the ratings systems that were our concern. Eventually, it became apparent that it was rather easy to turn our ratings into predictions. Each sport is treated very differently and to varying degrees of accuracy. As a starting point, we use the team ratings to formulate a base prediction.
Let’s look at an example as it applies to two teams:
We want to know who is going to win the 2012 SEC Championship which has been set between National Championship hopefuls Georgia and Alabama.
After all their regular season games in 2012, Alabama has a rating (x1) of 3.2712 and Georgia has a rating (x2) of 2.7662.
The formula for finding the probability of winning p(x1) is exp(x1-x2)/(1+exp(x1-x2)).
Translated into English, that means that the probability of Alabama winning the SEC Championship was 62.4%.
But there are so many other factors that might influence the outcome of a game: injuries, starting pitcher, weather, etc. We have added varying degrees of adjustments to different sports. Keep an eye out for these adjustments as we mention them on the sport’s prediction pages.