Monday, 25 November 2013

Pulisball. What to Expect at Palace.

Crystal Palace's long search for a new manager finally ended on Friday night, when one of the early front runners, Tony Pulis put pen to paper. Any lingering doubts about Pulis' at times ugly style of play was ultimately trumped by a managerial record that has never seen him relegated. Sacked yes, but not relegated.

Pulisball gained ridicule, contempt, grudging respect (and that was just from his own fans), but just enough points to guarantee perennial survival. With the unique ingredient provided by Delap's long throw it did prove an over achieving way of retaining Premiership status for a largely Championship quality team.

Long throws, long balls and set piece goals were the most visible face of a Pulis led Stoke, but it wasn't the only requirement if 40 points were to be achieved year on year. It also needed a defence. A side that relies on set pieces to provide a higher that average proportion of their goals are usually limited attacking sides. The correlation isn't overwhelming, but the trend exists for sides that score proportionally more goals from set plays, to also score relatively fewer goals over a season compared to sides that have a more balanced attacking approach.

Unsurprisingly, an inability to score goals also goes hand in hand with lower end of season points totals and the strength of the correlation is very strong. The r^2 values for success rate (a proxy for points) and goals scored since the start of the 2008 season is 0.77, so 77% of the variance in success rate comes about as a product of the variance in goals scored.

A slightly weaker correlation, but of a similar order of magnitude for r^2 also exists between goals allowed and seasonal success, which leads to the inevitable conclusion that goals scored and goals conceded by sides are themselves not independent. The negative co dependency between the number of goals a side scores and the number they concede is plotted below. The more goals a side scores in a season, the fewer they will tend to concede.

This correlation is probably driven, firstly by a general difference in relative ability between sides. Better sides are simply more adept at playing all aspects of football. Such sides score more and concede less, but there may also be a tactical aspect. It is difficult to score yourself when your opponents has the ball in your final third. In some cases, attack may also be an effective form of defence.

So the way a side either chooses or it forced to try to accumulate goals in the EPL can have an effect on the amount of goals you score. Stoke's reliance on set play goals was likely to result in few goals being scored. The poor goal totals would in turn lead to unimpressive seasonal success rates and that in turn often led to relegation at worst and bottom half finishes as the norm.

However, Stoke ended four of the five seasons under Pulis with higher seasonal success rates compared to the line of best fit for goals scored, denoted by the red points in the second plot. A clue to why this might have occurred can be seen in the plot above. In all years under Pulis, they allowed fewer goals than expected for a side that scored at the rate they did. Again Stoke's five seasons under Pulis are represented by the five red points in the plot above. The brutal simplicity of creating a set piece chance just by winning a throw inside the opponents half, often meant that Stoke didn't have to commit many players forward to do so. The tactical aspect of Stoke's chance creation left plenty of defensive resources intact to keep the score down in games that mattered.

We can't dismiss the possibility that Stoke's relative defensive excellence was just merely down to chance. If we look at enough sides, some are going to appear continually better than expected, but by pure chance. However, anyone who has watched Stoke, especially in the early EPL seasons can't have failed to notice the overtly defensive stance they took both at home and away from the Potteries.

Although they were correctly classified as a side that relied on set pieces for the majority of their goals, it would be a mistake to look at this aspect of their play in isolation. A team is a sum of their parts and the general outlook for set play sides is rather bleak. They are more likely to be relegated than a side that can score proportionally more often from open play. They finish in the bottom half of the table with greater frequency and they are highly unlikely to be capable of challenging for even a Europa spot.

Pulis' Stoke ticked most of the statistical boxes for a limited, set piece reliant side. Palace already score nearly half of their goals from that source, although raw numbers are small, so percentages will fluctuate. But the real wrinkle that Pulis introduced that propelled an approach with little upside or room for error to a relatively comfortable way of retaining your top flight existence, was his insistence on a defensive team effort.

Palace are already a side that find scoring difficult, so their limitations already provide half of the ingredients for Pulisball to make a Premiership return. To complete the formula, Pulis' secret sauce will be liberally applied to the defence in an effort to maintain his proud record.... but this time he'll have to do it already 12 games into a season and without a defence enhancing wildcard such as Rory Delap.

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