Saturday, 11 May 2013

Cranking Up The Goal Expectation When Doing Badly.

A football match is a contest that is constantly and subtly changing in many ways. Goals are the obvious major events that alter the balance by which teams either seek to consolidate an advantageous position or retrieve a potentially losing one. Goals come about through a combination of skill, random chance and no little effort and the varying degrees to which teams choose to attempt to impose this factors on an opponent determines how successful they will be. In this post  I looked at how trailing teams are more likely to score than they had been previously when they concede the lead.The amount of time remaining is also a contributing factor, but sooner rather than later every team will launch a concerted effort to retrieve a losing position. They don't automatically become the most likely team to claim the next goal, if there is one, but they do, on average become more dangerous in attack than had previously been the case.

The extra potency shown by such teams could previously only be quantified if their efforts produce a goal and over large enough samples their scoring rate when trailing can be shown to increase by upwards of 10%. However, by using models that predict goal expectations for individual goal attempts based on the x,y co ordinates from where they were made, we can demonstrate how sides, on average attempt to up their attacking game in certain match situations. Either until their opponents succumb, they themselves are caught on the counter attack or the game merely excitingly runs it's full course.

Arsenal, being a consistently successful side are less prone to ambiguous, stalemated game states, where doubt lies as to whether or not they are reasonably happy to be on level terms. Original game winning probabilities of around 25% or smaller are the break even point, whereby a side is theoretically content with a point and the vast majority of Arsenal's matches will see them quoted at greater probabilities than this to win at the outset. Therefore, Arsenal are almost certain to push for a winner at some point in almost every tied game unless they are visiting either Manchester club or Stamford Bridge.

Arsenal's Goal Expectancy from Chances Created in Various Scoreline States. 2010/11.

Scoreline State. Ahead. Level. Behind.
Goal Expectation from Chances Created. A Goal every 55 minutes. A Goal every 45 minutes. A Goal every 45 minutes.

The overall level of Arsenal's ability in 2010/11 was on par with a side expected to score, on average a goal every 51 minutes. The goal expectancy based on the quality and quantity of the chances they created when they led suggests that then they played like a team capable of scoring only once every 55 minutes. So, as a team which had the lead they moved into a move defensive mode to the detriment of their attacking expectations.

The Gunners' urge to improve during level and trailing scoreline states is reflected in their quantity and quality of goal attempts being the equivalent of a long term average scoring rate of a goal every 45 minutes. In 2010/11 they upped the rate of chance creation and partly maintained the quality in a level scoreboard state and upped creation even more, but at the cost of chance quality when behind.

In the absence of goals, we can still show the efforts, sometimes fruitless, made by Arsenal in losing or frustratingly stalemated situations. During the 79 minutes they trailed to Villa in their final home game of the season, Arsenal fired in enough goal attempts of varying quality to have scored at a long term rate of a goal every 30 minutes and their game long potential goal expectancy over the full 90 minutes was an equally urgent goal every 35 minutes. But the randomness of conversion rates saw them merely register a 89th minute consolation, despite their numerous efforts. They lost on the day, but through random variation rather than lack of trying.

Above I've plotted the overall, theoretical scoring rate suggested by all the chances created by Arsenal in each 2010/11 match against the average of the game state they encountered on the day. In matches where they were consistently chasing their hoped for outcome, they were able to up their attacking output, producing chances that would yield almost a goal every 20 minutes in their home loss to WBA and these bouts of increased effort often remain in the overall game figures. But, as with the Villa game, short term randomness again beat them.

At the opposite end of the plot, they were unable or unwilling to continue to take the fight to United and Chelsea when beating both. Defence was probably more of a priority once the lead had been secured. An early goal from the game's best chance against Wolves, allowed Arsenal to dictate much of the game at Molineux. More goals would have been welcome, but weren't essential and a second goal only arrived in injury time as Wolves pressed forward.

It appears that all sides eventually tailor their attacking intent to suit the current score, their own pre game expectations, the quality of the opposition and time remaining and if random distribution of your innate talent is kind enough to gift you a three goal lead at home to Chelsea, there is little need to try to run up the score at the risk of opening up the game. Scoring further goals no longer remains a high priority.

Single game scoring efficiency is a heady mix of match day randomness that infrequently yields significant, talent driven events and the relative abilities of the contestants. High quality chances often fail to result in a goal, whereas poorer quality ones sometimes do, and these partly random outcomes often help to frame how the remainder of the match is played out.

For more interesting work on this essential context driven subject check out Paul's recent post at differentgame.

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