When doing fits to my playoff formula while excluding Super Bowl data, I noted the following:

./logistic_game.pl --nosb --start=2001 --end=2015
b[0]   = prev playoff experience
b[1]   = strength of schedule
b[2]   = constant
b_p[i] = probability the term could be a product of chance.


Start year    = 2001
Ending year   = 2015
Data points   = 150
Home team won = 94

2 = Years checked for playoff appearance.
complete results
D0	198.211779799779
Dm	181.040960409088
Dm_chisq	17.1708193906915
Dm_df	2
Dm_p	[0.00018681164]
b	[0.93247135 0.25819533 0.74094435]
b_chisq	[ 9.5962352   7.944765  15.099918]
b_p	[0.0019497672 0.0048226681 0.0001019677]
iter	6

There are 5 playoff games every year in the NFC, and 5 in the AFC, and over the last 10 years, the home team has won 94 of them. Nominally HFA is usually assigned to be a 3 point betting advantage, and nominally that advantage is approximately 60%. 94*100/150 is 62.66666 repeating, which rounds to 62.7 percent. That’s more than the nominal 60% and more than the average of the last five NFL seasons. To calculate, 2012-2016 home field wins are 146, 153, 145, 138, and 147 respectively. That totals to 729 wins over 5*256 games. The percentage calculates to be 56.95%. Expressed in logits, and then in points, these advantages then become:

Home Field Advantage
Type of HFA Win Percent Logit Prob Calculated Point Spread
Playoff HFA 62.7 0.518 3.8
Traditional HFA 60.0 0.405 3.0
Seasonal HFA 56.95 0.280 2.1
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