Leading Indicators 2

"Anti-Predictors"

In the last post I compared the results of two regression models. The first model estimated current year wins based on current year stats. The second model predicted next year’s wins based on last year’s stats. The comparison of the regression results revealed how well various team stats persist from year to year as predictors of team wins.

I found that the stats that persisted from season to season as a predictor of wins were offensive running efficiency (36%), team penalties (52%), and defensive forced fumble rates (45%). I also found two stats that could be considered anti-predictors. Offensive and defensive interception rates both reverse their direction of prediction across seasons. In other words, a low offensive pass interception rate for a team in one year foretells fewer wins the following year, all other things being equal.

This is a confusing result, to say the least. One possible explanation is random coincidence--the stats may show a connection only by chance. However, the significance of the offensive interception coefficient was 0.07 and the defensive interception coefficient was 0.09. As I wrote earlier, the FDA might not approve heart medication based on trials with marginal significance levels, but it is still highly unlikely that both coefficients suffer from statistical Type I errors. After all, we’re not splitting the atom here. We’re just talking about football.

In the last post I wrote, “My theory is that we are witnessing regression to the mean. For many teams, interception rates have a lot of variance due to luck. So a team that is unlucky with interceptions one year is not likely to be as unlucky the next year, [and vice versa]. That could partially explain the reversed signs. Another possibility is that teams systematically swing from high to low interception rates from one season to the next, something I strongly doubt.”

A friend at work suggested I actually look at teams, their players, and what happened that might cause such a result. (What? There’s more to football than statistics?) I couldn’t bring myself to qualitatively analyze what might be going on, but he did inspire me to dig a little deeper.

Below is a list teams that demonstrated the trend of impressive interception rates one year, followed by a severe drop-off in wins the next. The average for both offensive and defensive interception rates is 0.32.

Year	Team	Wins	Next Wins	O Int Rate	D Int Rate
2002	TB	12	7	0.018	0.061
2002	OAK	11	4	0.016	0.037
2003	TEN	12	5	0.018	0.038
2003	KC	13	7	0.022	0.044
2004	PHI	13	6	0.020	0.031
2005	TB	11	4	0.029	0.036
2004	NYJ	10	4	0.025	0.038
2005	JAX	12	8	0.012	0.039
2003	SF	7	2	0.029	0.045
2005	CIN	11	8	0.026	0.060
2004	HOU	7	2	0.030	0.042
2002	ATL	9	5	0.025	0.047
2003	MIA	10	4	0.042	0.042
2005	DEN	13	9	0.015	0.033
2005	WAS	10	5	0.023	0.030
2004	BUF	9	5	0.037	0.049
2004	SD	12	9	0.018	0.038
2003	STL	12	8	0.038	0.047
2004	NE	14	10	0.029	0.037
2004	BAL	9	6	0.024	0.042
2005	SEA	13	9	0.021	0.028
2004	NO	8	3	0.030	0.024
2004	GB	10	4	0.032	0.015

These teams all exhibited a notably better than average interception rate on either offense or defense, only to suffer a dramatically worse record the next year.

Below is the list of teams that exhibit the opposite trend, no matter how slight. They exhibited better than average interception rates, then improved their record the following year.

Year	Team	Wins	Next Wins	O Int Rate	D Int Rate
2005	TEN	4	8	0.024	0.019
2005	NYJ	4	10	0.032	0.045
2004	NYG	6	11	0.027	0.030
2002	KC	8	13	0.027	0.029

Whatever the reason for the phenomenon, it appears real. It might be summed up by "Live by the interception, die by the interception." If a team wins a lot of games based on superior offensive (low) or defensive (high) interception rates, it tends to be extremely difficult to repeat, and that team will very probably not win as many games the following year.

In the next post, I'll apply the leading indicators to the team stats from 2006. I'll list how each team can be expected to benefit (or suffer) in 2007 from the leading indicators of NFL wins.