Thursday, 30 January 2014

Chelsea v West Ham. Not Quite A Typical 0-0.

In this blog, I've taken two slightly different approaches when looking at how game state alters the way teams balance their risk and reward approach to periods during a match. You can either primarily look at the best sides, which allows you to assume that a tied scoreline is almost always unsatisfactory for them and therefore scoreline can largely be used as a proxy for game state. Or more recently I've concentrated on games that remain scoreless and therefore, pregame odds can be used as a proxy for the overall game state experienced by each side.

On Wednesday night both of these approximations aligned as near top, Chelsea beat near bottom, West Ham 39-1 in shots, but only drew 0-0 on the scoreboard.

Chelsea were unsurprisingly strong pregame favourites, the hosts having about an 80% chance of winning and a 15% chance of drawing for an overall match success rate of 87.5%. So the longterm points expectancy for the title challengers from such a match was 2.55 league points, because they would win 80% of the games, gaining themselves 3 points and draw for a single point 15% of the time, leaving 5% left over for a shock away win.

The longer the game remained stalemate the further the expected reality on the night fell away from this hoped for average expectation. Goal expectation decays relatively slowly at first. As the clock ticked into the 40th minute, Chelsea could still expect to take around 2.3 points from West Ham, even though they had yet to make a breakthrough. Their points expectation after 40 minutes was still over 90% of what it had been at kick off. So there hadn't been particular cause to panic through the bulk of the first half.

 Above, I've plotted (in red) the rate and extent of Chelsea's declining pregame points expectation. 40 minutes in, as already mentioned, it had fallen to 90% of the original value at kickoff. But by the 90th minute, it had very nearly halved to 1.33 expected league points.

Superimposed on the graph I've included the goal expectation for Chelsea from each of their 39 goal attempts, based on the actual x,y location from where the attempt was taken. I've grouped the attempts into 10 minute slots.

Data from a single match is inevitably choppy, combined with sides perhaps playing in spurts of increased effort, rather than a smooth gradual cranking up of the pressure. However, the trend for Chelsea to become more intent on making a breakthrough appears to increase in tune with the decline of their game state. They produced enough individual goal attempts to find the net an average of nearly 3.3 times over the 90 minutes and they threaten more in general as the game wore on. Such was the extent that WHU became entrenched, that Chelsea's goal expectation from their actual shots taken in the second half approaches nearly three times their value from before the break.

The 39/1 shot ratio was exceptional. As noted here, superior sides, on average also have the lion's share of shots when a game ends 0-0. But typically, a side as superior as Chelsea are compared to WHU in terms of league placing should only claim around 72% of the total shots taken in the game.

Other splits, however were more typical, although they still indicate the excessively above normal rate at which Chelsea may have chased and WHU have hunkered down. Chelsea had 70% of the crosses compared to an expected 63%, and 74% of the total passes against an expected 62%.

Although the massive shot differential takes all the headlines and other attacking based ratios also indicate the severe imbalance between the mix of capability and intent on show from Chelsea and West Ham, the underdogs did have a minor "success" in the way clearances where divided on the night. Such large favourites might expected to only have to account for around 20% of the clearances made in the game, but WHU managed to force Chelsea into making over 30% of Wednesday's total.

Statistics for a single game are often determined as much by the in game situations each side is presented with, as by the relative gap in quality. Mourhino's side played a similarly limited side in Stoke on Sunday in the FA Cup. They didn't quite dominate The Potters in terms of shots, as they had done WHU, but they didn't need to, following Oscar's first half goal. After that it was just a case of keeping Stoke's relatively well disguised attempts at equalising at arms length and picking away with the regular opportunities that were available.

On Sunday, six goal attempts inflicted on WHU in injury time alone, simply wasn't called for against a Stoke side still playing possession football in their own half. A team produces a combination of what it can and what it needs to do and very occasionally, when the expected doesn't happen, one of those ratios goes off the scale.

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