Monday 19 August 2013

Walters and Lampard. Two Average Penalty Takers?

Picture the scene. The Liverpool players have ceased disputing the award of the penalty kick as Stoke's Jonathan Walters confidently places the ball on the spot. The Stoke fans are both elated and slightly apprehensive as the striker turns, before choosing power over placement by thumping the ball.....past the dive of Pepe Reina and into the dead centre of the goal.

Walters had opened his Stoke City penalty taking career by successfully converting his first Premiership spot kick at the Hawthorns in a customary easy win against the Baggies. His 100% record was slashed to 50% in the unforgiving way percentages can fluctuate in small sample sizes, when his next penalty kick was saved at Norwich. Success at home to Liverpool bumped his numbers back up to percentage levels that were closer to the league average and further converted attempts at home to first Newcastle and then Wigan finally propelled him to levels where he was above the league average for conversion rates.

Any thoughts that Stoke had an above average penalty taker where quickly dispelled when failure at home to WBA was overshadowed by the Baggies actually managing to win the match as well. A feat worthy of epic poetry for Stoke's long suffering local rivals.

By the time Walters had opened his penalty account in the 2012/13 season, once again against Wigan, he had been above the league average on two occasions, marginally below it thrice and well below it following three other attempts.

There then followed an afternoon of high farce, when the luckless Walters capped scoring two own goals for Chelsea with a missed penalty at the Boothen End. Stoke City fans and Petr Cech alike struggled to stifle a giggle. Even referees were joining in by now and he was required to re-take a successful and vitally important spot kick in the dying days of the season at QPR. However, heroic deeds and abject failures are at their rawest when they are most recent and an 89th minute miss at Anfield yesterday with the home side a single goal to the good was enough for Yahoo Sport to ask if John Walters, success rate 61%, is the worst penalty taker in EPL history.

It will be scant consolation to Walters, especially if he chances to log onto Stoke's Oatcake forum, to find that such Premiership greats as Dwight Yorke (60%) and Ces Fabgregas (60%) have inferior conversion rates and Stewart Downing fares even worse with 40%.

The Stoke player formerly known as Super Jonny Walters kicks the ball against Genoa.
So he isn't the worst....except the response required to such a blunt question can not produce as categorical an answer as the one demanded by the web giant. Walters' talent or as some may passionately argue through painful recent experience, lack of talent, hasn't been given the chance to express itself in a sample consisting of a mere 13 attempts. The fact that Walters has been above the historical average success rate of around 79%, slightly below it and substantially below it, depending on when we viewed his records should make us strongly suspect that, dependent on opportunity, there will be more major fluctuations in the Irish internationals headline conversion rate.

A mere 13 attempts is insufficient to use Walters' success rates in standard statistical tests to see if his record is significantly different from the average return expected from a true 79% penalty taker. We can take a slightly different tack and, either by simulated trials, as in the plot above or through a series of binomial trials, it is possible to estimate how likely it was that Walters recorded 8 successful kicks from 13 attempts occurred if each attempt had the generic 79% chance of being converted.

If we crunch the numbers there is just under a 7% chance that Walters is a perfectly average penalty taker who has just happened to score a below average 8 from 13. Ces Fabregas creeps into Yahoo's worst list with even less 12 yard punts. In recording a slightly inferior 60% conversion rate, he has converted 3 from 5 attempts and this time an average spot taker could record those figures 21% of the time.

So by accounting for the number of attempts players with near identical percentage success rates have been trialed over, we can accumulate extra levels of nuanced information that allows us to come to a better informed conclusion. Percentages alone always hide sample size.

Conversion rates are a perfectly adequate way to illustrate a players actual performance over a period of time, although success/ fail records are better. But they often do not reflect a players true levels of skill. That has to be couched in terms of likelihood and probability.

Seasoned penalty taker, Frank Lampard has a headline conversion rate in the EPL of 42 from 48, for 88%. A dead average kicker has a 5% chance of recording these inflated numbers. So if 20 average penalty kickers were asked to take 48 kicks under EPL conditions, on average you might expect one to record the same figures as Frank has done.

So there's a finite possibility that both Lampard and Walters are average penalty takers, exhibiting unlikely, but not precluded sequences.

Football's most valuable penalty was entrusted to Kevin Phillips in May, playing in the colours of Crystal Palace. Phillips' EPL penalty record of 11 successes from 18 attempts also makes the worst list, if we add back in the success/fail information, an average taker would fall to such conversion rates in around 4% of cases. His increased attempts make him a borderline case for seeing if his record is statistically different from that expected from a 79% converter.....and it isn't. Therefore, you could not statistically claim that Palace were entrusting a penalty in a stalemated playoff final to one of the worst penalty takers in his previous career as a Premiership player.

For Palace fans, this was the outcome.......Watford fans may wish to look away now.

Check out James Grayson's site for the best penalty posts on the web here
and for a detailed look at Frank Lampard, including his run of 8 successes from 13 attempts see Differentgame here

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