Tuesday, 3 March 2015

Brendan Rodgers' Post January Sorcery?

It's been a "good" week for stats driven narratives. Chelsea's lack of spot kicks, quickly followed by Depay's poor shooting and now Liverpool's post January spurt have all been inferred from mere accumulated data.

The latest story revolved around Liverpool's impressive points per game rate, post New Year compared to their form in the opening five months of the campaign under Brendan Rodgers' tenure.

The figures are neatly summed up below from the excellent source of Liverpool data, Andrew Beasley and although SkySports, to their credit added a ? to their headline describing the trend, twitter was immediately rife with theories behind Rodgers' inspired form in the second half of the campaign.

So are we again most probably looking at definitive conclusions being drawn from raw data, with little or no regard for the effects of random variation?

Only the initial two seasons are complete and although the games are split fairly evenly in the first two campaigns, that does not mean that the early and late games are of similar difficulty.This is most evident in the 2013/14 season.

Liverpool, of course finished runners up last season and in the first half of the season they traveled to play the teams that would prove to be their five closest rivals. Chelsea, Manchester City, Arsenal, Spurs and Everton.

A group of five difficult away fixtures followed, post January by the relatively easier reverse fixtures is likely to have made 2013/14's Aug to Dec fixtures more difficult than the following Jan to May contests.

If we take the implied probabilities from the bookmaker's odds, Liverpool were expected to average around 1.87 ppg pre January and 2.11 post 2013. Even allowing for an inflation of the Reds' rating as the season progressed, it is still likely that the second half of 2013/14 was easier than the first.

But before we attempt to provide reasons for this apparent split performance, (and the various absences of Suarez and Sturridge in early 2013/14 and again in 2014/15 add to the complex mix of potential interactions), we should first see how likely it is that the split appeared just by chance.

If we simulate the two seasons worth of split data and the currently unequal portions from 2014/15, around 1 in 5 trials result in three consecutive seasons where January onwards has been more prolific for Liverpool in terms of points per game compared to the previous five months.

Two seasons out of three where January marks an upturn in points is the most likely scenario, but none of the four possible combinations should be considered as highly unlikely.

This does not mean that Rodgers has not perfected the art of crafting his team into a more effective unit as winter turns to spring or he was unlucky with injuries, but the possibility that we are seeing a split of results that were highly likely to occur for someone, if not Liverpool over the last two and three quarter seasons, simply by chance, should be regarded as a likely contributing cause.

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