Saturday, October 31, 2009

Bye Weeks

Do teams have an advantage after a bye week ?
by Denis O'Regan.


There's been some interest recently on various blogs concerning the performance of teams after their bye week.So here's some number crunching I did a few seasons ago,update to the present.

I looked at the record of teams coming off their bye week playing against teams who had played the previous week.I eliminated late season weeks,such as week 17 where teams are resting starters or perhaps not fully committed to winning in order to secure higher draft picks.And I also stuck to the regular season to avoid divisional games where the,usually inferior opponent has played in the wildcard round.

I used win percentage as one measure of a team's success and also average margin of victory (or defeat).The latter measurement can add a little more depth to the process of measuring a team's performance.For example if a team wins three games each by a single point,it is 3-0 from a win/loss point of view,but has in all probability put up a very similar performance to that of its opponents.

I initially looked at home teams alone and then away teams alone coming off a bye and then also at away teams who were favoured to win their post bye matchup.

I compared the results of each post bye group with a much larger,but similar group of teams who were playing having played the previous week.

Lastly I have a win probability model for games that I have used for the last 10 seasons.Over the close season I have taken the process back another decade.Therefore I have used this to determine which team is favoured to win any matchup.The regression used to produce the probabilities does not incorporate any perceived advantage from a bye week.The expected average margin of victory for any group of matches can be calculated and if this expected margin differs greatly from the actual margin over a large number of games,it could be reasonable to assume that the difference could be attributed to the missing ingredient from the regression.Namely the effect of a bye week.



Results.

Case 1.
Home teams playing after a bye.
Wins 169,losses 120 for 58.5%.From the regression they were expected to win by an average of 2.9 points,they won by 3.0 points.

Compare this to home teams playing having played the previous week.
Wins 1815,losses 1304 for 58.2%.Expected margin of victory was 2.6 points,actual was 2.5.

There appears to be absolutely no advantage for home teams playing after a bye compared to their counterparts who did not have a bye week.

Case 2.
Away teams playing after a bye.
Wins 126,losses 141 for 47.2%.Expected margin of defeat was 1.7 points,actual was 0.8 points.
 
Compared to away teams playing having played the previous week.
Wins 1298,losses 1853 for 41.2%.Expected margin of defeat was 2.7 points,actual was 2.6 points.

A couple of points here.Firstly,the sample of bye week road teams had,on average slightly easier games than the control sample.Thus the 47% to 41% difference shouldn't be taken entirely at face value.However,the bye sample was expected to lose by an average of 1.7 points and it did almost a whole point better than that,losing by just 0.8 points.

Case 3.
Away teams as a whole look as though they may benefit from a bye.So I looked at various types of matchups,starting with away teams who went on the road as favourites.I used my regression equation to define the favoured team in the game and looked at away teams with a 55% or better chance of winning.The 55% was chosen to try to eliminate closer matchups where there could be greater doubt as to who was the favoured team.

Away teams playing after a bye who have a 55% chance or better of winning the game.
Wins 58,losses 14 for 80.6%.Expected margin of victory 6.0 points,actual 8.7 points.

Compare this to away teams playing having played the previous week who have a 55% chance or more of winning the game.
Wins 423,losses 201 for 67.8%.Expected margin of victory 5.7 points,actual 6.0 points.

Again a few points.The regression equation,which remember does not factor in an advantage for bye weeks,appeared to pick out two very similar groups before the fact.It expected each group to win by an average of about 6 points.(The Vegas line for both groups was similarly close).

The group that had not had a bye week performed pretty much as expected.However,the bye week group did considerably better than either Vegas or the regression expected,winning by almost 3 more points than expected on average and posting 81% of wins compared to 68% for the control.

To test if the effect was repeatable and not the result of a couple of freak seasons,I split the away favourites playing after a bye sample in two at the year 2000 and the effect was present in both groups (although sample size issues were obviously now present).

Conclusion.

Favoured teams going on the road after a bye week appear to overperform by almost a field goal.This effect is largely absent in all other types of matchups.

4 comments:

Anonymous said...

Do teams that have a bye later in the season gain an advantage? What is the win lose record of teams coming off a bye by week? I was under the impression that the later in the year a team has their bye, the better the record.

Anonymous said...

Where can I find a list of the bye weeks for each past NFL season? Was hoping to test this theory out.

Anonymous said...

Denis - I just sent Brian an email validation of your hypothesis from a slightly different point of view.

If you will give me an email ID, I will share with you also.

Regards - Jeff from Indy (jeffloop@comcast.net)

Anonymous said...

I would love to see that as well. The Ravens just became one more data point in the plus column. Could you email it to Matt@CompUnlimited.com, please? Thanks!

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