Monday 16 November 2015

Will Leicester Emulate WBA's Hot Start from 2012/13 ?

Goal expectation models have become increasingly popular in evaluating past performance and predicting future achievements, both for teams and individual players.

More widespread data availability has allowed models to grow in complexity.

Simply taking the cumulative goal expectation for a teams attacking and defensive units does shift the wheat from the chaff, but the methodology can be easily improved by looking at the goal expectation of individual attempts and allowing for multiple saves in the same attack.

How cumulative goal expectation is distributed over a game in terms of the number of chances and likelihood of success for each individual attempt can have a subtle influence on match outcome between two teams with a similar goal expectation in that match.

One use of granular models is to simulate the variation in possible match outcomes compared to the actual result on the day.

Such an approach is of course limited, matches aren't merely heading and shooting contests, but in a young discipline it may highlight which team is benefiting from variation in outcomes and which is not.

In short, teams are dubbed deserving or not of their current league position based on how closely their expected goals profile tallies with the league table.

WBA spent 2009/10 in the Championship and 2010/11 and 2011/12 as comfortable mid table Premier League finishers.

After 12 games of the 2012/13 season they were 4th, a point behind Chelsea, 4 adrift of Manchester City, who'd narrowly beaten them in injury time in October and 5 behind leaders, Manchester United.

Despite a recent history of Premier League mediocrity, their position on November 18th 2012 seemed legitimate. Their goal expectancy was just over 18 goals and defensively it was 13, very close to their actual goals record of 19 for and 13 against and they had just beaten Chelsea at home.

So 23 points from WBA's 12 games, based individual on goal attempts in those matches seemed to be a fair return.

However, if we split into groups the goal expectancy for all on target attempts in WBA's 12 games they were particularly dominant in non penalty chances that had at least a 0.5 probability of being converted.

And while such a distribution of attempts had served them well in reaching 4th spot, samples were small and it wasn't necessarily going to be typical of how the season might continue for WBA.

The two plots above show the typical distribution of non penalty kick, on target attempts in a season for teams who finished in the top 4, (where WBA stood after 12 games) and for sides who finished from 9th to 12th, broadly the position WBA occupied in their previous two Premier League seasons.

The attempt profile for the first 12 WBA games of 2012/13 appears to be more typical of a mid table side when chances have a relatively low individual expectation, but that of a top four team when chances had a higher individual goal expectation.

So in projecting the Baggies performance in the remaining 26 games, do we broadly take their 12 games to date as an indication of improved form, even if their dominance in big chances only amounts to a handful of attempts at both ends?

Or do we treat these mere 100 combined attempts as a possible aberration and assume WBA will perform more in keeping with their previous two seasons in the remaining 26 games and weight any projected ratings accordingly?

The goal expectancy profile for the remaining 26 games is above and WBA were largely out-shot during the remainder of the season in all but the lowest probability attempts, rather than maintaining their hot start.

They finished with 49 points in 8th spot, just two more points than they'd won in each of their previous two campaigns.

Two seasons of mediocre performance proved a better indicator of future success than did a 12 game sequence where WBA out-shot their opponents in high probability attempts.

Early season WBA in 2012/13 weren't lucky in converting their chances, but they may have been lucky in the distribution and number of chances they were creating.

Perhaps their Midlands rivals Leicester City, with a similar early season goal expectancy profile and lofty current league position, will fare better in 2016 than the Baggies did in 2013.

Or more probably, not.

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