The most surprising season in Premier League history? What are the odds…

It’s fair to say that the result of the 2015-2016 Premier League season was a shocking one, with Leicester City winning their first league title despite starting the season priced at up to 5000/1.
In all likelihood such a shock will never happen again if only because bookmakers will have learned the lesson of placing such steep odds.
This made me wonder though at how we can quantify the surprise-factor in individual games using the odds offered by bookmakers before the match starts.

The data available at football-data.co.uk might be able to offer some insight.
They provide match results (and other stats) but also odds on these results from various bookmakers for several years of English and European football.
Let’s start with Premier League games since the 2000-2001 season as from this point the odds data is consistently present with only a few gaps.

Which were the most surprising individual results?

As you might expect they are all away wins for unfancied teams against high quality opposition.
At the top of the list there’s Blackburn’s famous 3-2 win against Manchester United at Old Trafford which was the perfect storm of an upset result, with Blackburn sitting at the bottom of the table at the time, and with Man. United going top if they win. There was also the added embarrassment of this game occurring on Sir Alex Ferguson’s 70th birthday and, although they didn’t know it at the time, United go on to lose the title to rivals Man. City on goal difference, so it turned out that had they not lost this game they would have won the title.

We can also group these results by teams across each season, summing up the odds of them winning each of those matches that they won.
I was expecting to see Leicester’s 2015-2016 season appear top here, but it was beaten into second place by West Ham in the same season.

You can think of the ‘Win odds sum’ column as being the total amount you’d win if you bet £1 on this team win their game every week.
On average, this should come out at £38 (or a bit less accounting for bookmaker’s profit) for the season since you would expect to win back about £1 for every £1 spent.
So West Ham’s 2015-2016 value of 73 is very impressive.

Breaking that down by game:

Whereas for Leicester we have:

So even though Leicester are winning more, and still at some long odds, it is West Ham’s capacity for big away wins despite having a mid-table season which makes them the most surprising team (in terms of winning at long odds) in recent Premier League history.
Over the whole history of this dataset we can plot these cumulative winning odds:

The unusually low point last season was of course Aston Villa, while the all-time low point, in the 2007-2008 season goes of course to my own club Derby County, whose solitary win that year was against Newcastle.
Putting £1 on Derby to win every week would have returned less than a fiver across the whole season.

One last thing we might like to do here is try to quantify how surprising the season as a whole is, so lets take the variance of the cumulative winning odds of the 20 teams:

It’s pretty clear then that last season had by far the most surprising results of recent Premier League history – lets hope it continues.

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