Saturday, 29 October 2011

Are Championship Sides Coping Better in the EPL?

It's hardly inventing the wheel or the iPad to suggest that goal difference is a very powerful football statistic.The more goals you score as a team and the less you concede,then the greater your chances of winning in the low scoring environment that is football.We've seen here and here that average goal difference is both a very good indicator of past success and because it auto correlates strongly it is also an excellent predictor of future performance.

Goals are the currency of football.

The good early season showing of all three promoted sides has led to the predictable conclusion that the gap between the Championship and the EPL is steadily becoming more easily bridgeable.Both QPR and Norwich are currently in the top half of the table and the playoff  winning Swansea are only three points behind the duo.If the former Championship sides were hunting as a pack they would have amassed 33 points from a combined 27 games and be around seven points from safety with eleven games to play.An impressive performance all round,even when allowing for the natural tendency for over achievement in part of a season to be balanced out by slightly less impressive subsequent results.

Recent events naturally lead to a selective bias in the way people analyse events and information.East Anglians,even Ipswich Town followers will have Norwich's fine recent draw at Anfield at the fore front of their thoughts when discussing the EPL,rather than Ipswich's much more substantial achievement of winning playoff promotion in 1999/00 and following up with a fifth place finish in the EPL the following year.

The natural first step is to evaluate the quality of the sides making the journey between the two leagues and their average goal difference is a good tool.It quantifies a team's worth and is easily comparable to the goal difference they achieve in the higher league and although you won't be able to deduce much about the absolute quality of the team,the change in a team's goal difference will indicate how easily they made the jump up in class.

There have been just over fifty Championship sides promoted to the EPL since 1993,so I split the sample in half and compared the average goal difference per game recorded by teams in their promotion season in the Championship and their figures for their first season in the Premier League.

Average Goal Difference per Game Comparisons between the EPL and Championship. 

Season. Average Goal
Difference per Game in Championship.
Average Goal Difference per Game in EPL.
2003-2011 0.66 -0.57
1993-2002 0.67 -0.41

There is very little to chose between the goal difference per game of either group in their Championship phase,but promoted teams fared on average almost two tenths of a goal better in the initial seasons than they did in the most recent seasons in the highest tier.These numbers imply that on average the Championship sides found the transition to to EPL easier to make in the fledgling years of the Premiership.

To try to confirm this initial impression I then plotted  the individual  regression lines for each batch of games to confirm that there was a reasonable correlation between the Championship season and the EPL season and one or two particularly outstanding promotions weren't skewing the averages.Visually there does appear to be a correlation between seasons and the line of best fit is steeper in the earlier batch than in the later one.This appears to confirm that similar average goal differences in the 90's and early 2000's  translated into better goal differences in the first season in the EPL compared to the later batch.

As an example,the line of best fit in the early seasons of the EPL sees an average seasonal goal difference of 0.6 goals in the Championship translating into an average goal difference of -0.5 in the subsequent EPL season compared to  -0.6 goals in the later seasons.Once again earlier EPL years saw the promoted teams faring slightly better judged by the regression line's predictions.Also the average finishing position for promoted teams in their first year of Premiership soccer was 14th between 1993 and 2002,while from 2003-2011 they finished a place lower in 15th.

The goal difference from a team's promotion season would therefore appear to be a reasonable indicator of their likely performance in the higher league,so I then looked at how robust the relationship is for the different categories of promoted sides.The Champions could be expected to be the strongest contenders for future success as they have most likely proved themselves consistently superior to their fellow promoted sides over a 46 game period and if we use the EPL as a guide the most successful teams are also the most consistent.If you plot the goal difference comparison graphs for the Champions,the second placed sides and the sides promoted via the playoffs we find the correlation between seasons is much the strongest for the Champions and the strength of correlation falls for the second placed sides and is almost non existent for the teams emerging from the playoffs.

The correlation between goal difference gained in winning promotion to the EPL and subsequent performance is strongest for the Champions and weakest for those who had to win their Premiership passports at either Cardiff or Wembley.

It would be naive to believe that points gained in the previous season is the sole factor in determining subsequent points totals,there are mainly contributing factors,the most obvious one being money spent in the pre season transfer window.The shape of the points graph for Championship winners finely illustrates this missing factor.The linear fit for the points is relatively strong,however the best correlation is illustrated in the plot below.

The best fitting curve comparing goal difference pre and post a team's promotion to the EPL.

Four teams who averaged a goal difference of around 0.8 performed relatively badly once in the EPL.A significant number of sides who averaged a goal difference of just 0.6 goals actually acquitted better when they made the leap forward .If we look at the individual teams involved the four 0.8,yet underperforming teams spent virtually nothing in preparation for their EPL campaign.Nottingham Forest,steeped in a tradition where Peter Taylor bought a million pound talent for peanuts from Long Eaton United,sold more players by worth than they bought in their EPL season(and were promptly relegated),Sunderland spent a net £130,000 (and were promptly relegated),Norwich spent £3 million (ditto) and WBA spent £4 million and were relegated by 3 points.

By contrast the group of teams who performed slightly worse in their promotion year than the yo yo group  spent a net average of £18 million and that outlay enabled them to put in a significantly better EPL performance.Charlton spent £18 million,Middlesbro' £16 million,Sunderland £40 million and even the lowest spenders,Sunderland back in 1996 invested £6 million.The high rolling group also outspent the more successful Championship winners,who appeared to reason that their higher quality squad required a slightly smaller outlay in order to survive their initial foray into the top flight and the graphical representation of thir campaigns confirms that their optimism was well founded.The best of the Championship Champions invested an average of £10 million,half the amount spend by the ambitious,but weakest of the champions.The lack of investment,especially early in the history of the EPL is understandably as the risk of instant relegation,high wage bills and potential bankruptcy was very real,but that threat receded with the introduction in 2006 of parachute payments to the demoted teams.

So for the Champions at least transfer money spent and previous goal difference are both significant indicators of first year EPL performance.It's not as significant for runners up and playoff winners,who are coming to the post season transfer scrum somewhat later and could possibly find themselves buying poorer quality players at unattractive and inflated prices.

A Championship champion's likely EPL goal difference is given by

0.01 * (millions spent) + 0.85 * (average goal difference in winning the Championship) - 1.344.

If we therefore bring the article full circle and use the average figures for our imaginary,composite promotion entity comprising the combined records of QPR,Norwich and Swansea.The teams finished 1st,2nd and 3rd and within 8 points of each other,so for this mild diversion I'm going to treat the teams as Champion class,even if only QPR have the silverware to prove it.We find that they would expect to end with a EPL goal difference of -0.71 given their outlay so far and past goal difference.Currently their combined goal difference measured over their combined 27 games is -0.48,so as a group they are finding the EPL marginally easier than their record and the record of all previous Champions would have expected them to.However,the smart money would be on their individual records falling back to earth come May,although almost certainly not as dramatically as the decline experienced by Hull in their 2008/09 season,when they joint topped the table,were unbeaten in all but one of their first nine games,won at the Emirates and White Hart Lane and yet ultimately escaped relegation on the last day of the season. 

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