Tuesday 30 August 2016

The 2016 NFL Regular Season Done & Dusted in Excel.

The NFL’s back and so are the LA Rams, so here’s how I'd go about modelling the 2016 season.

Firstly, you need a rating for each team in the new season.

Previous season’s data is always a good starting point, but the NFL is a relatively short 16 games regular season, so wins and losses from 2015 can be heavily influenced by luck or random variation to give it a less provocative name.

Winning lots of games by narrow margins, winning more or fewer games than your points scoring/conceding merits and feeding off lots of turnover ball are usually big indicators that your win/loss record may regress towards the league of 8 wins in the upcoming season.

These traits are often described as "knowing how to win", but they are almost always just random......or possibly cheating.

Based on how teams with these flags against their record did in the subsequent season over the last decade, the Carolina Panthers (15-1) would expect to regress to around 10 wins in 2016. 

The four win Chargers should be dragged upwards to just over 7 wins.

These projections are around where the Panthers and the Chargers are quoted in the season total wins markets. So we’ve got a decent starting point that’s potentially stripped out some of the unsustainable luck that went into the 2015 regular season.

Here's the predicted wins for all 32 teams based on last year's luck based indicators.

Next we need a way to predict individual match results.

The NFL is blessed because we have Bill James’ Pythagorean Log 5 method, which takes winning percentage and home field advantage and spits out the odds of a home and away win. 

(It looks like this =((E2)*(1-G2)*$B$1)/((E2)*(1-G2)*$B$1+(1-E2)*(G2)*(1-$B$1)), where E2 is the home team win%, G2 is the away team win% and B1 is the home winning %, currently around 0.57).

The Panthers are projected as a 10-6 team, so 10/16 or a 0.625 team.

You put estimated win% for each team into each match up and get the estimated home win% for the match as the output.

Do this for every regular season game. 

The projected 0.39 Rams go to the 0.27 SF 49ers on Monday night, week one. 

Last season the Rams were in St Louis and if we stick our regressed, new season win% into Bill’s formula we get LA 1.8 (a shade of odds on) to win on the money line in SF or a spread of around 3 points in LA’s favour.

LA are currently quoted at -2.5 favs, so again, we’ve got a decent model.

Next we need simulate all 256 games from week one to week seventeen.

Again it’s easy in excel. 

We judge that LA will win in SF with a probability of 0.56 based on our money line odds of 1.8 and if we stick a random number between 1 and 0 alongside this estimated win probability and if it falls below 0.56, we grant LA their win, otherwise it’s a SF win.

Again we can do this for every regular season game and we’ve simulated one NFL season for 2016.

Ideally we’d like to repeat this a couple of thousand times and once again even excel obliges in less than a minute with a decent computer.

Check out this soccer post for the basic method.

We now have a range of season wins for all 32 teams, based on a reasonably robust new season rating and their actual intertwined schedule.

Seattle has the best projection in 2016, they are expected to average just over 11 wins.

However, the simulations illustrate the range of outcomes that are possible even for the likely best team in the NFL just due to the randomness in a short 16 game season.

From the plot of the outcomes from 10,000 iterations of the 2016 season, there is an 8% chance that Seattle will not have a winning season. Small, but certainly not insignificant.

More positively, there’s around a 1 in 1,000 chance they go 16-0. Bookies will currently give you around 499/1

It’s around 200/1 that someone goes 16-0 in 2016, again you might get 66/1 on that.

Ratings can be updated as the season progresses or you can add your own tweaks at any time, such as subjective adjustments to account for current rosters.

Finally, here's the finishing probabilities for the NFC West, based on 10,000 league simulations and appropriate tie breakers.

Fairly close to the quoted odds for the top two, with the Rams and 49er's shortened on the books in case either "do a Leicester".

Seattle and Arizona are most likely to also nab the top seeding in the NFC, along with Cinci, New England and Pittsburgh in the AFC. But you could have probably guessed that.

The Jets have about a 14% chance of winning 12 or more games, which might be an AFC outsider worth running with. 

While Dallas and Detroit each has a 5% chance of doing likewise in the NFC and getting a high post season seeding.

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