Examining the NFL: An Empirical Analysis of
Common Beliefs in the NFL
Dennis Lock
Data Set
• Data collected from various location all
available through NFL.com
– Team offensive numbers
– Team defensive numbers
– Starting quarterback numbers
– Team turnover numbers
• Data for each team from the 2008 season.
Goal
• Use this data to examine and critique a set of
four common statements.
•
•
•
•
“Defense wins Championships”
“If you win the turnover battle you win the game”
“A prolific Offense must convert on third downs”
Differences between the AFC and NFC
Wins
“Defense Wins Championships”
• Measure defense with Yards per game against.
“Defense Wins Championships”
• Graphically examine the relationship between yards per game
allowed and wins.
“Defense Wins Championships”
• Compare this to the relationship between yards per game for
and wins.
r = -0.725
r = 0.571
“Defense Wins Championships”
• So defense appears to be very important for winning games.
• The best defensive team ended up winning the super bowl.
• So there is evidence that defense is very important in order to
win a championship.
“If You Win the Turnover Battle You Win the
Game”
• Turnover differential is the number of takeaways from the
opposing team subtracted by the number of giveaways to the
opposing team.
Turnover Differential 5 Number Summary
Minimum
1st Quantile Median
3rd Quantile Maximum
-17
-6
+5
0.5
+17
– The maximum of +17 was achieved by the
• Miami Dolphins (11-5)
– The minimum of -17 was achieved by two different teams.
• Denver Broncos (8-8)
• San Francisco 49ers (7-9)
“If You Win the Turnover Battle You Win the
Game”
• Graphically examine the relationship between turnover
differential and wins.
r = 0.43
r = 0.017
“If You Win the Turnover Battle You Win the
Game”
• So this idea is not clear, for average and below average teams
it appears winning the turnover battle may not lead to more
wins, but it does appear to be important to make the playoffs.
• So perhaps a better statement would be: “You must win the
turnover battle to make the playoffs”.
“A Prolific Offense Must Convert on Third Down”
• Third down is the last chance to keep the offense on the field.
• Analysts say this is the most important down for a team to
have success.
• In order to determine if this is true we can compare percent
success rate on third downs to offensive and defensive
statistics.
“A Prolific Offense Must Convert on Third Down”
• Again we will use yards per game as a measure of offense, and
compare this to third down conversion percentage.
“A Prolific Offense Must Convert on Third Down”
•
Analysts will often go on to say converting on third downs helps the defense.
– Again yards per game allowed will be used as a measure for defense.
•
There does appear to be some
relationship, however it is fairly
weak ( r = -0.309).
•
Adding number of wins as point size.
- Very bad teams could not do
either well.
•
Removing teams with 4 losses or less
we get a r = 0.088 (positive and not
significantly different from 0)
“A Prolific Offense Must Convert on Third Down”
• For offense this appears to be correct, there is a strong
enough correlation to provide significant evidence that for an
offense to be good it must convert on third downs.
• However it does not appear converting on third downs helps
the defense.
Differences between the AFC and NFC
• Unlike baseball the rules are exactly the same between the
AFC (American Football Conference) and the NFC (National
Football Conference).
• Announcers still discuss and believe there are differences
between the two conferences.
Table of means for NFC and AFC
Conf
Wins
YdsG
YdsAG
3rdP
Penalties
Giveaways
Takeaways
Sacks
AFC
8.125
322.1
326.5
40.1%
173.0
23.75
25.5
30.0
NFC
7.8125
332.4
328.0
38.9%
185.4
25.8
24.1
34.8
Difference
0.3125
-10.3
-1.5
0.012
-12.4
-2.05
1.4
-4.8
Conclusions
• “Defense wins championship”
– Evidence shows that this is an accurate statement.
• “If you win the turnover battle you win the game”
– Conflicting evidence, it appears you need to win the turnover battle to
make the playoffs.
• “A prolific offense must convert on third downs”
– Evidence shows that this is an accurate statement, however converting
on third downs does not appear to help defensively.
• Differences between the AFC and NFC
– There appear to be no differences between the NFC and AFC.
Future Research
• Examining other seasons, and observing if the same
conclusions hold.
• Examine multiple seasons at once to observe trends.
• There is an almost unlimited number of ideas like these four
which can be examined empirically.
Thank You, any questions?