The title of this post is a little misleading, I’ve taken an oft-used disclaimer ‘past performance is no guarantee of future success’ which is legally enforced throughout the investment industry (and beyond) to remind customers that if an investment product has performed well before, it doesn’t necessarily follow that it will continue to perform well.
Nevertheless, the majority of our decision-making and forecasting does rely on historical data or experience which means that in reality past performance is almost always a significant factor in prediction. How much importance you place on the historical record, combining this with information from [ideally] a wide range of reliable sources is up to you the analyst.
Today I’ve used data from the last 13 seasons (you may remember some of the charts I put together in this post) to give a sort of calculated guideline to next season’s final Premier League table.
First up, an introductory graph:
The graph shows, for teams who were in the Premier League for consecutive seasons between 2008/09 – 2012/13, the change in total points from the previous season: 63 data points for those wondering. Teams generally, it seems, don’t improve or reduce by much more than +/- 15pts from year to year and about 80% of the time stay within 10pts of their total from the previous season.
Of course there are exceptions, like Liverpool’s 2009/10 season where they dropped 23pts and 5 places from the season before or Newcastle’s season just finished: -24pts and -11 places. The biggest improvements came from Newcastle again (this time 2011/12) +19pts, +7 pos and Spurs in 2009/10: +19pts, +4 pos.
Goal difference tells a similar story, this time the 80% range is about +/-20 goals.
There’s a hint of negative skew to both charts but I doubt I have enough data to determine if that’s significant. I’d suggest it’s probably something to do with the top 7 teams’ domination of the league – arguably it is easier for the rest of the league to drop points against the top teams and the gap to widen between 8th and below than it is for 8th and below to make gains.
But what can we do with these historical results that might be worthwhile? Well, here are some images from my spreadsheets to try and give an idea!
Here I’ve reformatted some information on the past position of each team in the Premier League since 2000/01. No Cardiff here, but every other team in the league next season has played a part for at least 1 season over the past 13. Having said that, I don’t realistically think that Crystal Palace’s solitary 04/05 season will actually be a good predictor of their season to come – then again – 18th place… hmm.
I’ve put some averages in there for all you regression to the mean addicts and some adjusted average & standard deviation figures. Standard deviation isn’t relevant for most teams because you need lots of data points for it to be worthwhile but I’ve put it in there anyway! Based on this I’ve calculated an expected league position range in the far right column. The ranges are quite wide for the mid-table and below teams, frankly because historically there has been a small spread of points between 8th and 18th.
I’ve done the same kind of thing below for points and goal difference expected ranges:
Now, of course, how important you think history is a factor in future results will determine how much you are interested by the expected results in these tables. You may wish to make certain additional adjustments if, for example, you think that perhaps West Ham or Southampton are being undervalued by the historical record – or perhaps Stoke are being overvalued now that Pulis is no longer their manager.
However, I’d say it’s a reasonable base set of information to which as analysts we can use Bayesian inference to consider new information like injuries to star players in certain teams, new signings that increase the expectations of the team’s outlook, the impact of a new manager on a team’s results and so on.
The teams that this analysis considers most prone to relegation (judging by the points table) – in addition to the newly promoted teams – are Sunderland, Aston Villa, Southampton and West Ham.