Monday, 8 February 2016

The Shawcross Redemption.

Stoke fans are restless again.

A couple of weeks after eagerly anticipating a Wembley trip and making a push for a domestic cup double, settling for 4th place in the Premier League and regular trips to the Nou Camp, while fighting off the advances of Chelsea and Manchester United for the services of Mark Hughes, they've seen all this fade to grey.

They're now wondering how Hughes could be so inept as to neglect Stoke's defensive and attacking frailties in the January window and when he should go to avoid regular trips to the Pirelli Stadium, Burton.

Knee jerk punditry has nothing on those with an emotional and financial investment.

Top of Hughes' current rap sheet in the eyes of his previously greatest supporters is his neglect of adequate cover for the central defence.

It probably doesn't help that on the same weekend that Robert Huth (allowed to leave on a free) was visibly winning the title for Leicester, Stoke captain Ryan Shawcross was again sitting out a tame home defeat to Everton through another injury.

With Huth & Shawcross in tandem, Stoke often dispensed with the services of a keeper.
With or without stats are horribly blunt devices, where small win/loss samples can "prove" bit players essential to a side's well being, when they are merely the coincidence with little causation.

But just as action or heat maps for a single match for more numerous in game stats may shed some light on where a side won or lost a single game, aggregating such things as the quantity and quality of chances allowed with or without a particular player may also tell us something about their impact.

Shawcross has obligingly missed about half the season through injury and the odd suspension. Here's the expected goals per 90 that Stoke have allowed when he's played and when he hasn't in the Premier League. (excuse the familiarity in the table, it was done for a Stoke fan site).

So there is tentative evidence that a player who has had his fair share of media scrutiny has become an important, if not irreplaceable part of Stoke's defence.

His most obvious attribute is his strength in the air and his ability to prevent attackers winning the aerial challenges (often, and in keeping with many defenders, by anchoring the attacker to the ground by his shirt tails).

If you look at the number and proportion of headed chances conceded when Shawcross is and isn't on the field again the contrast is marked. 20% of the total attempts come from headers in his absence of which five have resulted in goals compared to nearer 10% when he plays, with no goals conceded.

It has been suggested that the skill differential when players use their head is greater between players than when they use their best foot, so for once the fans may actually have a point when they debate that Stoke have one of the Premier League's best defensive headers of the ball, but nothing in the way of cover.

Friday, 5 February 2016

Putting Your Best Foot Forward.

Finishing skill has been an acknowledged fact of football virtually forever. Strikers are never more dangerous than when they are being "clinical", "ruthless" or, for those of a certain comic strip vintage, "Dead-Shot".

Unfortunately this almost mystical ability has constantly eluded every effort to pin it down even as the data generally available becomes more extensive and plentiful.

It is relatively easy to find strikers who are under or over performing their expected goals model based on any number of shot location variables, but persistence of this trait is less obvious.

Often the "cold" player from one month/week/match/half is the same "hot" scorer from a similarly recent time frame.

   The Magical Finishing Skill Aura of "Dead Shot" Keen's boots worked for Billy Dane. 

Shot volume and location can usually be relied upon to produce an expected goals figure that tracks fairly well a player's actual goal tally. But expecting even a season-long over performance to extend to a subsequent season (at least with a rudimentary model) is often a forlorn hope.

Random variation or rare or unlogged events, such as deflections and defensive pressure appears to overwhelm any attempt to observe a quality that is currently worth around 2 billion Chinese yuan.

A player may differ in finding space, receiving passes and anticipating where to be inside the box, but it is likely that the difference in finishing ability once the chance presents itself is going to be small between the elite.

Marginal gains, but also expensive mistakes if luck is purchased masquerading as a repeatable talent.

The biggest talent gap in finishing skill at the top level should lie between strikers and the rest of the outfielders.

So I looked at every shot (headers excluded) taken by every oufield player in a chance created solely from open play, which wasn't deflected and created an expected goals model based simply on the location of the shot. Sample size well into five figures.

Unsurprisingly, the location of the attempt in this sanitized shooting competition was a significant indicator as to the likelihood of a goal being scored.

I then told the model which shots were taken by "Dead-Shot" strikers and which came from the boot of non-strikers. The expectation being that this additional variable would prove significant and improve the likelihood of the strikers scoring at the expense of their team mates who were less talented at finishing (or they would presumably be strikers themselves).

It didn't.

In this dataset, knowing that a striker had taken the shot slightly decreased the likelihood of a goal, but this effect had almost certainly arisen entirely by chance. The model couldn't see a difference in the likely outcome regardless of whether the shot came from a defender or a striker.

If there is a difference in finishing ability between Premier League outfield players in different positions, as opposed to other desirable attributes possessed by a striker, a naive shot location model can't cut through the missing variables and noise to find it.

So instead I looked for a set of Premier League shots that should/might be (much?) less likely to be scored than others and could be picked up by a simple shot location model.

Scorcher's Billy Dane aside, most players don't have magical football boots, but they do have a preference for one foot over the other. I've yet to find a penalty taker who hasn't taken all his kicks exclusively with a particular foot.

Regular penalty takers used their penalty taking foot for nearly 80% of their shots from opportunities created in open play. So you also have to think they know something about the "finishing ability" of their standing leg.

I re did the model.

Again in the model shot location was a significant variable in the outcome of the shot. But this time when I added a variable for whether the shot originated from the player's penalty or non-penalty taking foot, that too was (almost) significant.

Benchmark figure, a shot with a player's "weaker" foot reduces the chances of a goal by around 10% of the value if it had if it had been taken with his penalty kick foot.

Every player demonstrates finishing ability and that difference might show itself on the 20% of occasions he uses his "swinger" and hits and hopes.

Thursday, 4 February 2016

"...And Then We Went To The Etihad".

Manchester City entertain surprise package Leicester in the mid day televised Premier League game on Saturday in the first of five, potentially high leverage head to head matches involving the current top four teams between now and May.

It is unusual to have four teams in genuine contention for the title with just 140 matches remaining, so although the outcome of the early kick off will move the dial it won't be as dramatic as if there were fewer title hopefuls.

The current market odds favour Manchester City followed by Arsenal, the respective second and third favourites in the preseason. So August liabilities may be still skewing the market's February estimation of either lifting the title.

By contrast, Tottenham and Leicester where available respectively at triple and quadruple digit odds.

Numbers are oblivious to any monetary balancing of the books and even the fluctuating levels of future performance that a high profile manager in waiting may inspire. They simply rise or fall as the matches are played out.

Not so very long ago, Leicester were just Championship FA Cup cannon fodder for the Premier League Big Boys.
Manchester City has averaged 1.83 expected goals per game and allowed 1.09 in the season so far compared to Leicester's 1.58 and 1.21 respectively, which gives the hosts a 53% chance of winning, 23% the draw and 24% the visiting Foxes.

The market is more bullish about the hosts (five Premier League losses so far) beating the twice defeated upstarts. It puts Manchester City's chances at nearer 60%.

There will be around 20 minutes to digest the result from the Etihad before the probabilistic projections of Spurs entertaining Watford and Sunday's trip to Bournemouth by Arsenal begin to turn into real points.

There'll also be ample time for the North London fan base to root for the best case scenario for their respective sides in the early game.

So how will the three possible outcomes alter, not only the title chances of the two Citys, but also those of Arsenal and Spurs?

How a Manchester City win might change the title odds at 3 o'clock on Saturday Feb. 6th.

How a draw might change the title odds.

How a Leicester win might change the title odds.

Obviously a win is the best possible outcome for either Manchester City or Leicester.

The host would draw level with their visitors with a win, the most likely outcome. Viewed purely in terms of the relative strengths and remaining schedule of the four challengers, Manchester City's likelihood of winning the title would remain below 50%. Although  in a potentially skewed market they are likely to move to odds on.

A Manchester City win is also marginally the worst outcome for Arsenal.

Spurs can root for a Man City win or a draw. Although the latter would turn their Valentine's Day game at the Etihad into a high leverage game.

A Leicester win would eat into the chances of each of their three competitors, particularly Manchester City's.

Although their underlying inferior defensive and attacking expected goals would mean that even a six point lead would be insufficient to overturn a title win by someone other than the Foxes as still the most likely outcome come 3 o'clock on Saturday.