Wednesday, January 8, 2014

Year to year volatility in NFL standings

Question:   To what extent was team record in the NFL in 2012 related to team record in the NFL in 2013?

Short Answer:  There was a fair amount of change in the performance of many teams from the 2012 to 2013 seasons.

Data:   The raw data used for this analysis is presented below.

Team
2012
2013
2012 minus 2013
AT
0.813
0.25
0.563
AZ
0.313
0.625
-0.312
BA
0.625
0.5
0.125
BU
0.375
0.375
0
CA
0.438
0.75
-0.312
CH
0.625
0.5
0.125
CI
0.625
0.688
-0.063
CL
0.313
0.25
0.063
DA
0.5
0.5
0
DE
0.813
0.438
0.375
DR
0.25
0.813
-0.563
GB
0.688
0.531
0.157
HO
0.75
0.125
0.625
IN
0.688
0.688
0
JA
0.125
0.25
-0.125
KC
0.125
0.688
-0.563
MI
0.438
0.5
-0.062
MN
0.625
0.344
0.281
NE
0.75
0.75
0
NO
0.438
0.688
-0.25
NYG
0.563
0.438
0.125
NYJ
0.375
0.5
-0.125
OA
0.25
0.25
0
PH
0.25
0.625
-0.375
PI
0.5
0.5
0
SD
0.438
0.563
-0.125
SE
0.688
0.813
-0.125
SF
0.719
0.75
-0.031
SL
0.469
0.438
0.031
TB
0.438
0.25
0.188
TE
0.375
0.438
-0.063
WA
0.625
0.188
0.437


Note that some teams, which did very well in 2012, did very poorly in 2013.  These teams with a disappointing year in 2013 included Atlanta, Detroit, and Washington.

Other teams (Arizona, New Orleans, Philadelphia, and Kansas City) improved a lot and exceeded expectations.

Some Analysis:  Well how closely is 2012 performance associated with 2013 performance?  One measure of this association is the correlation coefficient.   A correlation near +1 indicates that on average teams that performed well in 2012 also performed well in 2013.  A correlation near  -1 means a good year in 2012 is associated with a poor year in 2013.

I used Excel to calculate the correlation of percent of victories in 2012 with the percent of victories in 2013.   The correlation between these two variables is  -0.003, pretty close to zero.  Essentially the correlation statistic reveals no relationship – either negative or positive -- between record in 2012 and record in 2013. 

My next approach involved categorizing team records for the two years into two categories -- less than or equal to 500 and greater than 500.  
I have included the 500 clubs in the lower group.

Theses two-by-two tabulations are presented below.


Observed 2012 and 2013 outcomes
<=500 in 2013
>500 in 2013
<= 500 in 2012
11
7
18
>500 in 2012
8
6
14
19
13
32


The results indicate that

7/18 (38.9%) of teams with a record that is 500 or below in 2012 had a record over 500 in 2013.  This is pretty close to the (13/32) 40.6% of teams in the entire sample with records greater than 500.

8/14 (57.1% a majority) of teams with a record of over 500 in 2012 had a record that is less than or equal to 500 in 2013.  This is reasonably close to the 19/32 (59.4%) of teams in the entire sample with records that were 500 or lower.

My bottom line is that between 2012 and 2013 there was a fair amount of year-to-year volatility in NFL standings.

Additional Work:  How has the volatility in year-to-year records of the NFL changed over time? My perception is that volatility has increased.

How does the volatility of championship outcomes vary?   The Baltimore Ravens (the defending Super Bowl Champions) did not make the playoffs.  But several teams that went fairly far in the playoff last year – Denver, New England, San Francisco and Seattle – are still in the hunt.  It is possible that the 2013 NFC championship game – Seattle versus San Francisco – will be a repeat of the 2012 championship game.

Finally, how does year-to-year volatility in standing and championship outcomes differ across sports?  My sense is that football is more volatile than baseball or basketball.












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