Is There 'Momentum' During a Season?

The 2006 season continued and I made incremental improvements to the model. Beginning at week 8, I emphasized the efficiency stats from each team's most recent 4 weeks. I hoped this would help account for a team's improvement over the course of the season and for injuries to key players. I set this to be a changable variable which I called an "emphasis factor" that I could set to 0 to see how much the predicted probabilities changed from the baseline.

What I found was that the probabilities changed, but not by much, and rarely enough to swing one team from underdog to favorite. It made intuitive sense to me, and still does, so I kept the "emphasis factor" in the model.

But this is actually a question ripe for research. Teams appear streaky. Commentators, analysts, and fans accept that teams can be on a roll or in a rut. But over the course of a season it would be reasonable that wins and losses can naturally come in bunches on occasion.

Just as if you flipped a coin 16 times, you wouldn't expect the outcome to perfectly alternate heads and tails. If you did this several times, you would find that some of the 16-coin-flip "seasons" have several instances of consecutive heads "streaks." If we were betting on heads, our human intuition would tell us that the coin is "hot" or on a hot streak. In some rare, but totally natural, cases we might even expect 8 straight heads followed by 8 straight tails, or vice-versa.

The same principal holds for sports. The 16-game NFL season is particularly short and is susceptable to natural "streaks" without any actual change in performance. In theory, an average team destined to go 8-8 could start the season 8-0 followed by an 0-8 "collapse." Admittedly this would be extremely rare. In this case the luck factor (calculated below) for this team would be +4 during the 1st half of the season, and -4 for the last half of the season.

This phenomenon may explain why the emphasis on recent performance I applied in the model didn't change the probabilities very much. Teams appear streaky in terms of wins and losses, but win/loss records are naturally more erratic than the efficiency stats, so we should expect teams to appear that they're on "a roll" or in "a rut" when winning or losing streaks are really just part of a natural distribution of outcomes.

Below is a table of Week 11 comparing the predicted probabilities using both the standard stats and the stats with the recent 4-weeks performance weighted.