Rob Minto

Sport, data, ideas

Tag: Chelsea

Chelsea’s lack of penalties is completely normal – here’s why

466393940Chelsea took the unusual step of publishing an official moan about their lack of penalties this season. It has been widely reported (Guardian, BBC), but without anyone really taking them to task on the data. But a little statistical digging might have shown that they have nothing to complain about.

The Chelsea article said:

It is in our 28 Premier League games this season where we have been awarded just two penalties. Both were for infringements on the league’s most-fouled player, Eden Hazard, and both were in home London derbies, against Arsenal and QPR respectively. The most recent was four-and-a-half months ago.

Historically, this figure seems abnormally low.

In the Double-winning 2009/10 campaign, when we were the country’s outstanding attacking team, we were awarded 12 league penalties.

So let’s look at the evidence. The numbers that Chelsea point to only look at their own penalties awarded. Statistically, it’s known as sampling bias, but you don’t need to know that to see that it is a bunch of numbers out of context.

What we really care about is a few things: how many penalties should a team expect over a season? Are better teams given more penalties? And how do the league winners compare? The only way to know this is to (with apologies to Peter Moores) look at the data.

Chelsea did indeed get 12 penalties in 2009-10. But this is an outlier – in fact, for all the penalties data I could get from the 1998-99 season onwards, it is the highest number given to one team in a single season.

Two other teams have also been awarded 12 penalties in one campaign. Can you guess which teams they are? Have a go. Other league winners? Nope. In fact, it was Liverpool, in 2013-14 when they finished second; and Crystal Palace, in 2004-05, finishing in 18th place!

That might give a clue as to whether league position and penalties are connected. Basically, they are not. They are very weakly correlated, by a score of -0.28. *

Over a season, the average penalties per team per season has varied between two and six. And in the 16 years of available data, the Premier League winners have had a lower penalty count than the average team five times. That leaves 11 times when it has been higher (see chart below). Yes, you would expect the league winners to play attacking football and get a more penalties than the league average, as Chelsea suggest – but for Chelsea to get less than the average this season is hardly unprecedented.

EPL penalties

Put another way: only four times in the sixteen years of data have the team winning the league also been awarded the most penalties (Arsenal 2001-02, Manchester United 2002-03, 2007-08 and Chelsea 2009-10). Penalties are not some divine right of the best team. History shows that a team can be given a lot of penalties and still finish low down the league. Just ask Palace. Or Sunderland (6 penalties, 14th place last year). Or Blackpool (8 penalties, 19th place in 2010-11). Or West Ham (9 penalties, 17th place in 2009-10).

In other words: Chelsea’s current lack of penalties is nothing strange. It’s just… football.

* A negative number should be expected here, as a better league position is a lower number. For penalties and league position to be correlated, a score closer to -1 would be needed. For those wondering, it is very weakly positively correlated to the points a team gets over a season, with a score of 0.32.

When it comes to FA Cup upsets, size is subjective

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Bradford: biggest shock EVER?

What’s the biggest FA Cup shock defeat ever? For Robbie Fowler, it’s the Chelsea 4-2 loss to Bradford from yesterday (Jan 24). And in terms of drama, it’s clearly a great story. After all, Chelsea were 2-0 up and at home.

But for league placings, it’s not even close. With Chelsea top of the premiership, and Bradford 7th in League 1 (the third tier, despite its name), there are 39 teams between them.

Compare that to the 84 teams between Blackburn and Oxford in 1964, which according to Steve Porter, author of The Giant Killers website is the greatest FA Cup upset ever. Blackburn were 2nd at the time in the top division; Oxford were 18th in league 4.

Porter, who writes under the name Captain Beecher, ranks the upsets in terms of league placings, combined with a player quality metric using internationals and previous cup winners. Porter doesn’t spell out his methodology, but it’s clearly better than just using collective memory and non-scientific lists published in newspapers.

Porter sums up the problem perfectly:

On BBC’s Match of the Day programme, when asking the public if Bradford’s victory over Chelsea was the greatest cupset ever, they showed twelve of what they considered the greatest giant killings of all time. Every game had one thing in common. The BBC TV cameras were there. Not one game which was not covered by the BBC was considered. And so shapes our opinion. If you’re told something was a huge giant killing enough times {7th placed top flight Wimbledon beating Champions, Liverpool 1-0 in 1988. Surprise? yes but giant killing? Really? 7th vs 1st in the Premier League?} You start to accept that it’s true. ITV are a little more impartial, perhaps because they don’t have as much cup footage to be able to make lists exclusively thiers. The problem when compiling such lists is that every time a particular tie is overlooked, it’s chances of being placed in the next TV countdown, or magazine article diminishes.

And where does Porter’s system put Chelsea-Bradford? It’s 15th on his all-time list. Not bad, but it is interesting that it is lower down than non-league Luton’s 1-0 victory over the Premiership’s Norwich only 2 seasons ago, which is in 7th place. That didn’t even make the BBC’s list in the studio analysis. Memories are short, eh?

Being subjective, turning round a 2-goal deficit to 4-2 at Stamford Bridge is an extraordinary result. But perhaps the BBC could, with all its resources, dig up a few proper stats like Porter’s.

See also:
Interactive football league tables

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