Thursday 22 September 2016

Expected Goals and Game State.

The aim in competitive team sports is to score more goals or points than you allow your opponents.

However, often the route taken is subtly compromised by the ultimate intention of simply winning the contest and scores are neither maximised. nor scores allowed minimised.

More so in higher scoring sports, such as American Football, a side will react to the efforts of a trailing team by allowing yardage and possibly points to be scored against themselves in exchange for the trade of another valuable commodity, namely time run off the clock.

In short, team's react to the current score or game state and the multitude of statistics generated in this phase of a match may not be a true representation of the gulf in quality between the teams in a more equally balanced phase.

"Garbage time" touchdowns when already trailing by four scores may alter our assessment of the abilities of two teams in a more favourable way for the defeated side.

Therefore, it is commonplace to assess an NFL team based on the numbers they record when within one score either side of a tied game and further restrict collection to include pass/run neutral downs and distance, such as 1st and 10.

Football has fewer scoring events than its Stateside cousin, but game state and performance, particularly if measured in expected goals, may benefit by dicing the data to similarly include events that occur within one score of a tied scoreline.

Stoke have made an uncomfortable start to the season where conventional wisdom saw them as capable of taking the next step as a regular top ten and potential top six side.

Instead a run of actual results that more typically reflect their core expected goals figures from the latter half of 2015/16 finds them bottom with a single point.

Last Sunday's match at Crystal Palace highlight's how game state may produce expected goals figures that might not fully reflect the relative team merits.

Stoke shaded Palace in expected goals, but Palace scored four without reply until injury time.

In the admittedly brief period during which the game was level or within a single score, the home team had four attempts to none from City.

Just under half of Stoke's goal attempts and 65% of their total accumulated expected goals came in the final 15 minutes, when the hosts already led 4-0.

"Give Me Hope, Joe Allen"
Perhaps Palace considered conceding four in the final 15 minutes to a team who had failed to score at all in the first 75 was so unlikely they could coast to full time with little risk, save for a narrowing of the virtual divide and the odd real life goal conceded.

Maybe they slipped into the Premier League equivalent of a prevent defence, but few would argue that Stoke's expected goals "victory" over the 90 minutes hid a miscarriage of justice.

Palace out "expected goaled" Stoke with the scores level, when up by one, when up by two and when up by three. Whereas Stoke only dominated when the match was over as a recognisable contest.

With this in mind, here's the ranking of the expected goal difference for all 20 sides in the Premier League for all 2016/17 games and corrected for strength of schedule. Both with the game close and then for whole game data.

     Ranked Expected Goal Difference in All Matches and while Games are Close, 2016/17.

Liverpool have the best expected goal difference counting every minute played this season, but current leaders, Manchester City have been the most dominant in terms of expected goals created and allowed whilst their matches have been at their most contestable.

No comments:

Post a Comment