An excursion into ranking the NBA with Elo


P.S: Copy of my blog in Linkedin

Ranking and odd making are one of the oldest professions, probably dating to around AD 69 – the romans applying inferences on predicting gladiatorial shows! Fast forward, the recent NBA finals have become more interesting (from a Data Science perspective, of course) after the Cavalier’s Win !

Update (for those who were here before) : The Game 4 win by GSW (See the Update section at the end) shows how Elo adjusts for larger Margin Of Victory without oscillation!

One interesting algorithm is the Elo ranking, which has seen application in chess, computer games, NFL, NBA and Facebook ! In the movie Social Network, Eduardo Saverin writes the Elo on the glass, responding to Mark Zuckerberg’s call for the algorithm – the picture says it all !

An Introduction to Elo:

Leaving Eduardo and Mark Zuckerberg aside for a moment and moving on to the world of LeBron James and Steph Curry, Elo ranks teams or individuals in chess, basketball, computer games et al. The rank goes up or down as one wins or loses.

If a team is expected to win and it wins, the Elo rank goes up by a small amount; the gains are higher when a lower ranking opponent wins against a stronger team, with adjustments made for the margin of victory.

After every season, the rank reverts to a norm of 1505 (for Basketball) – but basketball teams being stable Year-to-Year, the folks at 538 has a distribution of 75% carry over and 25% revert to norm – we won’t deal with this now, but I did check this in my R program

Back to the main feature … NBA

The current NBA is a dream series for Elo – the thrills and chills of Elo can be observed! viz. a good matchup, but definitely a seemingly strong team, winning 1st game as expected; and boom, games 2 & 3 won by the (not so) weaker team !

You can see (below) the Elo stats at it’s best viz. capturing the transition, giving credit (and higher ranking) where it is due

I had done Elo for NFL, but wasn’t going to try NBA after game 1, but now lit ooks like a good exercise in data algorithmics …

Fortunately Nate Silver & his team has curated the basketball data from 1946 and explained their methodology. Thanks Guys.

I downloaded the data and did some R programming.

An ugly graph plotting Elo rating for the 2015 season for GSW (black) & CLE (blue).

We can definitely see that GSW is the stronger team, but CLE (Cavaliers) is getting stronger recently – especially as it wins over stronger teams.

Let us trace the stats summary ie the Elo rating of the teams, the point spread predictions, the actual score and the response from the algorithm ….

Stuff that brings tears to the eyes of a Data Scientist !

  • Going to Game 1, Elo said – GSW : 1802; CLE : 1712 ; Point Spread : GSW by 6.78 points. Actual – GSW by 8 points
  • Nothing fancy; the Elo ranking of GSW goes up by a little, CLE goes down a little
  • Going to Game 2, Elo said – GSW : 1806; CLE : 1708 ;Point Spread : GSW by 7.07 points. Actual – CLE by 2 points
  • Now, Elo kicks in ! CLE gains higher Elo (because they won over a stronger team), GSW loses more
  • Going to Game 3, Elo said – GSW : 1798; CLE : 1716 ;Point Spread : GSW by 2.92 points. Actual – CLE by 5 points
  • GSW’s Elo goes down; CLE’s future brightens; GSW still has a slim lead
  • Going to Game 4, Elo says – GSW : 1791; CLE : 1723 ;Point Spread : GSW by 2.3 points ! <- We are here (June 10,2015)

I will update with more Elo stats after Games 4,5,6 & 7 … (am sure it has the possibility to go to 7!)

6/11/15 : See Updates below

Incidentally Nate Silver’s tweets have an unintended consequence ! They are motivating Steph ! I am hoping this is the beginning of GSW’s path to a title …

Updates:

  • [Update 6/11/15 10:31 PM ] Actual : GSW by 21 Points !
  • Nate’s Tweets worked !
  • It is instructive to see the Elo graph. Even though the point spread (21 points) is much larger (than 2 & 5 points from earlier games) the Elo doesn’t go up by a huge amount. This is good, because we don’t want Elo to oscillate, but still should account for the larger than normal point spread. The Margin Of Victory multiplier adjusts that. Interesting to see the graph below, as Nate says it, in one game GSW regained their old position.
  • Going to Game 5, Elo says – GSW : 1810; CLE : 1704 ;Point Spread : GSW by 7.35 points (with home court advantage-refer to the formula (above) for details) ! <- Now, we are here (June 11,2015)
  • [Update 6/14/15] Game 5, GSW by 13 !
  • Going to Game 6, Elo says – GSW : 1814; CLE : 1701 ;Point Spread : GSW by 4.04 points (without home court advantage-refer to the formula (above) for details) ! <- Now, we are here (June 14,2015)

Reference:

  1. http://blogs.mercurynews.com/kawakami/2015/06/09/steph-curry-awakens-in-game-3-its-too-late-its-not-enough-but-its-exactly-what-the-warriors-need-the-rest-of-the-nba-finals/
  2. https://doubleclix.wordpress.com/2015/01/20/the-art-of-nfl-ranking-the-elo-algorithm-and-fivethirtyeight/
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