Saturday, 7 May 2011

Weighting


I've had worse days. AS Roma's draw with AC Milan at 3.5 and Everton's come from behind defeat of Manchester City were probably the highlights, on a day when every EPL team scored either one goal or two.

Everton's win was also tipped up by Football Elite (Draw No Bet) but they had a loser with Sporting Gijon.

Every single EPL Over / Under favourite lost, giving me three winners at 2.06, 2.03 and 2.14 from five selections. With the spreadsheet now using goal scoring probability to calculate match odds, it also gives me prices on the peripheral markets such as the Correct Score and the Over / Unders. The biggest value on Over was Newcastle v Birmingham which I had priced at 1.6, yet 2.14 was available. 2.03 was available for Overs at Bolton, a game I had priced at 1.65. West Ham v Blackburn wasn't so good - I had Over's here at 1.46 and available at 1.85, but we're not going to win them all. The prices on the Tottenham Hotspur v Blackpool game were almost exactly as I had - 3.15 available on Under where I had 3.17.

The excellent Soccermetrics blog has a post today which covers the subject of how to weight the data used to calculate a team's goal-scoring probability. I do this myself, since it seems to me that a goal at Chelsea should count for more than a goal at home to Blackpool, and similarly shots, shots on target and corners should be adjusted similarly. The weighting I use is based on the teams Elo based ratings, but Soccermetrics suggests using "the league position of the opposing team at the time of the match". He goes on to say that

I use a somewhat more sophisticated function. The rationale is the same -- goals scored against the top teams in the league should count more than goals scored against the cellar-dwellers. Or to put it another way, goals scored against lower-ranked teams are worth less than those scored against higher-ranked teams, and goals scored against the top team in the league (at that time) should count most of all.
No argument there, but I find the use of league positions rather simplistic and somewhat flawed. I left a comment which included:
The method I use is to rate teams using an Elo based rating system. This takes into account the strength of the opponent so that for example a 0-1 loss at Chelsea doesn't impact a rating so much as a 0-4 defeat at home to Blackpool. There are problems with simply using league position. Some are 1) Early in the season, these often are very misleading. 2) Games played do not always remain in sync. 3) A couple of points can separate several teams. 4) Conversely, a team can be well clear, or well adrift, but they are treated the same as if they were ahead, or adrift, on goal difference.
Soccermetrics has a far greater understanding of statistics than I could ever claim, so incorporating this idea into his calculations should be no problem.

For anyone interested, the Under / Over value tomorrow and Monday in the EPL looks to be on the Overs. I have Wolves v WBA at 1.58, with 1.89 available. Stoke City v Arsenal 1.63 and 1.91 respectively. Manchester United v Chelsea 1.93, 2.23 and Fulham v Liverpool 2.11, 2.23.

2 comments:

dannyjohnson1 said...

Off topic but think I might have spotted a little opportunity for you after todays man u/chelsea result.

Lay Man U 1st, Chelsea 2nd at 1.18 in the premier league scorecast.

Back chelsea to beat Newcastle at 1.27.

Thats a 8.55% ROI, barring bizarre goal difference permutations of chelsea winning the league.

What do you think

Jon said...

Maybe I'm missing something but what happens if Chelsea don't beat Newcastle but still finish second?