

Friday, February 4, 2011

Packers and Steelers fans, enjoy this Sunday

-- because "We'll be back next year" isn't so likely any more.
by Jim Glass

Once upon a time, way back before 1998, a team playing in the Super Bowl could reasonably expect to be in the thick of the competition for the Lombardi Trophy again the following year.

From 1970 through 1997 the Super Bowl team conference champions won an average 79% of their regular season games, and during the following year won 69%. In the follow-up seasons they compiled 47 winning records against only six losing and three .500 records. As 69% is 11 wins in a 16-game season, the teams could feel confident that they'd be in the playoffs again.

Then in 1999 the defending Super Bowl champion Broncos and runner-up Falcons, after going a combined 28-4 during 1998, fell to 6-10 and 5-11 respectively.

From then on, defending conference champions not named "Patriots" have had only 10 winning records in 20 seasons, against nine losing and one .500 record. All 24 Super Bowl participants including the Patriots have compiled a following-year average 56% winning record, a tad below 9-7.







               SB Teams W - L%               Next year season record











 SB yearNext yearwinning.500losing
1970-97 0.786 0.692 47 3 6
1998-09 0.784 0.560 14 1 9


Is this just a fluke among the small number of Super Bowl teams or a league-wide pattern? To see, I took a simple measure of regression to the mean for playoff-quality teams -- all teams that won 11 or more games in a 16-game season or 10 or more in a 14-game season -- and applied it to all of them back to 1970.

The measure is just the percentage by which a team's record retreats towards .500 from one year to the next. For instance, if a team is four wins over .500 in the first year and the next year it wins two fewer games it regresses by 50%; with five fewer wins it regresses by 125%, if it wins one more game it regresses by -25%. Call this measure "win regression".

Over the 12 years 1998 to 2009, teams that won 11 or more wins regressed by an average 75%. (The dominant teams that won 14 or more games regressed the same 75% as the lesser-winning playoff teams). And the trend has been up -- over the last three seasons the figure is 83%. That's a lot. With win regression of 83%, the next 16-0 team will be expected to go 9-7 the year after.

It seems that 1998 was a swing year of some sort. Because one-year samples of 11+ win teams are so small as to be highly erratic, I took five-season moving averages of win regression (the year plus two before and two after). For 1997, by this measure, win regression was 49%. The next year it jumped to 62%, and it's risen steadily since to the current 75% (or 83%), an increase of 50% (or 66%).

What's behind this? My first thought was the obvious one: free agency and the salary cap. With many more players moving, and teams having more equal resources, good teams logically would seem to have a harder time staying on top.

Bill Walsh's 49ers famously had both Joe Montana and Steve Young at QB for six full seasons. When Montana got hurt Young went in. But no team can afford two such players at the same position today, and players that good won't stay to sit. When Tom Brady got hurt after the 16-0 season the Patriots had to send in a backup who hadn't started a game since high school (and when he played OK they had him bid away). If instead they'd had another Hall of Famer to send in as Tom's replacement, their "regress" the following year might have been a lot less than to 11-5. So it seems that free agency and the salary cap are a very logical explanation for a big increase in win regression.

But they came into effect in 1993, five years earlier -- and that was at a prior peak of win regression. From 1989 through 1992 win regression ran at 70% to 73% -- and in 1993 it started falling. By 1997 it was down to 49%, a decline of near a third. That doesn't fit the "free agency and salary cap" explanation at all.

What else to say? In the 1970s win regression ran at only 30% -- it was a lot easier to maintain a dynasty in those days. After that, a picture probably describes better than words.


The vertical line on the chart is 1992, the last year before free agency and the salary cap.

The overall trend is clear, one can draw a straight line from 30% in 1972 to 80% in 2010. But the ups and downs may seem odd because they don't at all fit the story of "parity", as evidenced by win regression, being caused by the salary cap and free agency. The steady rise in win regression started a good two decades before then. When free agency and the salary cap did arrive, win regression plunged for half a decade. After which, for no obvious reason, it started heading back up again to higher than ever.

What explains these turns in the curve? I have no idea, except perhaps simple randomness in annual numbers around the trend line. (The chart doesn't look unlike a stock market chart.) The number of 11+ win teams each year is quite a small sample.

What's been the force behind the rising trend line in win regression, if it hasn't been free agency and the salary cap? There are a lot of possibilities, you probably can come up with at least as many good ones as I can.

But the final result of it all is that we can expect win regression of 80% or more from this year to next. So the word for Packers and Steelers fans is: Carpe diem!

 
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