by Bruce D
There have been some recent discussions at the Advanced NFL Stats main web-site concerning Atlanta and San Diego, with varying theories about how much luck is involved in their current win records. In a nut-shell, its why is San Diego's win record so much lower than their GWP, and why is Atlanta's win record so much better than their GWP. One of the prevailing theories is its due to bad and good luck(turnovers etc.). The other theory is poor or efficient coaching/skill.
Maybe its a combination of both. But what ratio? To try to answer this I looked for consistency with “luck” in each game, for each team this season.
First a little about how luck is tracked, here's an excerpt from a previous post.
“Team luck points = (bad luck points)-(good luck points), so negative numbers are the luckiest
For a more in-depth explanation of what "luck" points are, go to a previous post here.
Luck is tracked to better analyze a team's true ability and to help predict results of upcoming games where some may not know what portion of a team's record and points performance was due to just luck.
Luck points are valued as follows:
Points for(+) the unlucky team, are the same amount of points against(-) the lucky team.
field goal miss/block=2.5
punt returns for a TD=4.5
ko returns for a TD=4.5”
So now that we know how luck is tracked, and using the 2 teams mentioned as an example, this is what was uncovered.
First for San Diego:
Next for Atlanta:
This first thing you may notice is that Atlanta's week to week luck seems smooth and consistent, where San Diego's jumps around erratically.
The next step was to measure variance in a statistical manner, so I went to wikipedia here, and ran the variance formula for each team this season. The results are below.
Now all kinds of things pop out. Atlanta and Tampa Bay have a very low variance of luck. Is it good coaching, is it skill? I'm starting to think a lot of it may be.
San Diego and New England have the lowest and highest amount of luck respectively and they both have high variances of that luck. So should we consider that's its mostly due to luck?
I'm not a trained statistician, so I don't know what a 56.1 vs 6.2 variance really means, but it's easy to see that one is a lot, and the other is small.
If anyone has the time to comment in layman's terms what the variance numbers might represent (percentages?, avg fluctuations etc..?) it would be appreciated. For now I just assume its an indicative value.
In closing, I'd like to say, I now think Atlanta IS that good.