Thursday, 6 July 2017

Game State Outliers

Newcastle's 2011/12 season remains one of the most interesting of recent times.

They scored just four more goals than Norwich, but gained 18 more league points and allowed two fewer goals than Stoke and won 20 more points.

Their meagre +5 goal difference was inferior to the three teams who finished immediately below them in the final table and a 5th place finish was partly down to the hugely efficient way in which they conceded and scored their goals.

The ability to leak goals only when a game was already lost and score at the most advantageous times proved transient and the following season Newcastle's elevation to the top tier of the Premier League stalled as they barely finished above Sunderland and relegation.

In this post I looked to define game state in terms of not simply the current score, but also the equally important factor of time elapsed.

The current state of the game for a side is a combination of the score line, the relative abilities of each side and how long remains for either team to achieve a favourable final outcome.

As an example, take Stoke's home game with Everton.

The matchup was fairly even, Everton the better team being balanced by the Bet365 stadium and after 6 minutes the hosts had around a 37% chance of winning and a 25% of drawing.

That equates to an expected league points of 1.4.

A minute later Peter Crouch scores to put Stoke 1-0 up and their expected points with 93% of the game remaining and a goal to the good, rises to 2.1 league points.

The goal's welcome, but mitigated by the large amount of time remaining and the evenly matched teams.

No VAR and the game ends 1-1.

The plot above has averaged the increase in expected points per goal scored in an attempt to see which sides were scoring goals that most advanced their potential expected league points, either by design or raw chance, combined with their core ability.

It shouldn't be surprising to see the better teams having the lowest average expected points improvement per goal in the Premier League.

They are more likely to win matches by large margins and the 4th goal in a rout will add little to the teams expected league points, which will already be close to 3.

However, even among the top teams there are variations.

Spurs have the lowest expected points increase per goal scored, partly due to wide margin wins against the lesser sides, while Chelsea, with a similar number of goals, found themselves celebrating a score with, on average, a more tangible game state reward.

Hull appeared to occasionally put themselves into relatively decent positions, despite meagre scoring, while Sunderland, not only scored fewer goals, but also frequently only netted when the spoils had largely been won by their opponents.

The same point may be better illustrated by plotting the success rate ( a combination of  wins and draws for each team) against their expected points increase per goal scored.

Chelsea are apparent outliers from the line of best fit, scoring goals that advance their game state, on average  by more than their fellow top sides.

Again this might suggest that they are employing slightly different in game tactics compared to others.

Perhaps one that deserts further attacking intent for a more defensive outlook once they find themselves in a favourable match position, as do Manchester United....Or perhaps there is an element of random good fortune in when they are scoring their goals, a la Newcastle 2011/12.

Both Championship enigmas, promoted Huddersfield and their beaten playoff rivals, Reading, show anomalies from the seasonal norm when we examine their change of expected points based on goals and time elapsed.

Huddersfield fly high above the general line of best fit for a side of their scoring capacity, fed by a glut of goals where the time factor had nearly ebbed away. Again, tactically and skill driven or transient good fortune or a bit of both?

Reading showed an uncanny ability to know instantly when they were beaten by "selectively" leaking many of their 64 goals allowed in a handful of games "allowing" themselves to spread the remainder  of their concessions more thinly and remain competitive in a large number of their matches.

A "trait" that will be eagerly anticipated for their 2017/18 season.

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