Saturday, September 8, 2012

How accurate are football preseason polls?

I'm a few weeks late on this post, but I think its still worth blogging about.  Every year, the media (especially ESPN) makes such a huge deal about college preseason football polls.  Without any games yet to be played, these polls are little more than speculation.  I wanted to look into how accurate these polls are at choosing that season's national champion.  In this post, I will be using exclusively the AP poll preseason and final results, which can make a difference in years before the BCS when there could be multiple national champs based on the poll used.

This first plot shows the final ranking of preseason top 5 teams since 1990.  The bar furthest to the right shows the teams that were in the top 5 preseason poll but finished the year unranked.

 A few interesting notes:
1. 14 of the past 22 national champions were ranked in the preseason top 5 (I think the last winner outside the top 5 was Auburn led by then-unknown Cam Newton).
2. More national champs were ranked preseason #2 than preseason #1.  This is good news for Alabama who started this season ranked #2 behind USC (but who jumped to #1 after their first win).
3. Looking only at the preseason #1 teams (blue bars), they are more likely to finish the season ranked 3rd than any other rank.  Also, no preseason #1 has finished worse than #16 in the final polls.

Next, I wanted to look whether teams ranked higher in the preseason poll tended to be ranked higher at the end of the season.  A simple way to do this is to look at the median finish of the top 5 preseason teams.

Median Final Ranking since 1990
Preseason Rank    Median Final Rank

So although the national champions are not always ranked preseason #1, the top preseason teams in general finish higher in the standings than the other preseason teams.  The exception is that teams ranked #5 tend to finish the season ranked better than the #4 preseason team.  There could be some bias causing this result, as there needs to be some tie between teams with the same final season record, and this may be influenced by the preseason rankings.

Finally, I wanted to see how the final rankings of the previous season influence the preseason polls of the next season.  For example, do teams who finish the year #1 tend to be the top ranked preseason team the following year (even though 1/4 of the team likely graduated)?
This plot shows that the preseason top 5 teams tended to finish the previous season ranked highly.  We see that, since 1990, 9 of the 23 teams finishing the previous season #1 were the top ranked preseason team.  This is a somewhat questionable strategy, as only 2 teams have repeated as national champs since 1990: Nebraska in 1994-95 (who was not ranked preseason #1 in 1995) and USC in 2003-04.

In summary, while the top preseason team more often than not does not win the national championship, on average they finish the season ranked better than any other preseason team.