Friday, April 22, 2011

Goal Difference crucial for Premiership survival

The relegation battle this term in the Premiership has become increasingly intense. With Manchester United seemly stumbling towards the title as Arsenal and Manchester City reel off-course, the bottom of the table proves now to be the more intriguing sub-competition; a race no club or fan wants to win.

As we examined last week which Championship clubs from may take their place among English football's elite, the scrap for who replaces them in the second tier is in full flight. Seemingly European candidates two months ago, Sunderland are dropping like an action-movie elevator, while Blackpool's astonishing start to the season came undone at about exactly the same time as Charlie Adam's Liverpool move was rejected. On the other hand, Wolves have proved the most plucky of all the teams in the relegation zone yet still prop up the table, hit hard by injury to target-man Kevin Doyle.

What confuses this situation more than in years past is that there are no "certainties" for the drop. Last year Portsmouth failed to break twenty points (thanks among other things to a nine-point deduction for going into administration) and in 2008, Derby County broke Sunderland's record from 2006 for the fewest points in a season. There's no such luck for clubs hovering outside the zone this year - this season there are no easybeats. The entire bottom half of the Premiership table sits within one "six-pointer" of the drop zone.

When comparing this season to the previous six, there really is no precedent to the tightness in the relegation battle we see this year. In every other year, with the exception of 2009, there has been one club cut adrift at the bottom of the league. In 2009, that club was West Bromwich Albion, who rallied mildly at the tail-end of the season to finish with the same points tally as nineteenth-placed Middlesbrough.

Another trend over recent years has been that the tighter the relegation battle has become, the more impact Goal Difference has on which clubs survive. In 2007 and 2008, eight clubs each year finished the season within six points of relegation - or one crucial win against a fellow straggler. Excepting Derby County in 2008 (who finished the season with 11 points and an all time goal difference record of -69), it's easy to see that the average Goal Difference of relegation-threatened clubs decreases as the number of clubs "in trouble" increases. We've defined "threatened" as a club within six points of the drop zone.


Clubs within 6 points of relegation zone

Average Goal Difference of threatened clubs

2010-11 (5 or 6 games remain)











-36.67 (incl. Derby County)

-18.71 (excl. Derby Cty)










Derby County can be excluded because they are a statistical outlier - their season-long goal difference a whole 60% worse than any club's during the past seven years. Since they lost almost every game (season record 1-8-29) we can assume everyone took points off them. This assumption may not necessarily be correct, but statistically speaking, it is safe.

As you can see, the tighter a relegation battle gets, the tighter clubs tend to become - with the possible exception of Ian Holloway's Blackpool. If more club become involved in a relegation battle, it leads to lower average goal differences across those threatened teams. This season has produced another statistical anomaly which is interesting (but not very interesting) - Mark Hughes' Fulham are the only "threatened" club in seven years to boast a positive goal difference (+1).

Therefore, we can say safely with approximately 85% of the season complete, the 2010-11 goal differences figures are (on average) probably going to be the lowest of the past seven years. Extrapolated, these mean figures could be as low as -15.9 over the course of the entire season. If we use Goal Difference as a marker of how intense a relegation battle is, then this relegation battle is (statistically) about 20% more intense than the previous most intense one in 2008, involving Birmingham, Reading, Fulham and Bolton.

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