| Retrodictive | Predictive |
|---|---|
| Tries to get best fit to current season's data | Tries to produce ratings useful for predicting future games |
| Determines which teams had the most impressive season | Determines which teams are strongest |
| Does not publish predictions | Might publish predications |
| Does not use data from previous season | Uses data from previous season early in year to avoid bad predictions from insufficient data |
| Early season ratings don't mean much because of lack of data (some systems don't publish early in the season for this reason) | Early season ratings are meaningful |
| Has no preseason ratings because there's no data | Might have preseason ratings |
| Might be based on win/loss record only | Systems serious about predictions usually use the margin of victory and maybe home field advantage |
| Judged by how well past results are fit. For example, what % of games were upsets based on the current ratings. | Judged by how well the predictions work. For example, what % of games were upsets based on the ratings from the day before the game. |
Predictions are for entertainment purposes only.