A good goalkeeper keeps his side in the game, and there are none better in the Premier League than West Ham's Jussi Jääskeläinen - at least according to the statistics.
The Hammers have the most efficient defence in the English top flight - an impressive 4.91-shots-on-target-per-goal-conceded. Basically, against West Ham it takes an opposition more shots on target to force a goal. The higher this number then, the more efficient your defence.
And Jääskeläinen must take some of the credit, because the Finnish goalkeeper has been an ever present for Sam Allardyce's side this term. But no sane person would argue the West Ham stopper is the best keeper in the Premier League - so there must be another explaination for West Ham's impressive ratio.
We've listed the top 5 best and the top 5 worst Premier League teams based on shots-on-target-per-goal-conceded ratio. And then we've looked at one from each group in a little more depth to try an explain what it all means. Why do Manchester City have one of the worst defensive ratios despite conceding the least amount of goals? Why do West Ham possess the most efficient defence? Why does 'Big Sam' Allardyce persist with an eye-wateringly boring style of football? All will be revealed, good sir.
Now, shots-per-goal-conceded is not a perfect measure of the best and worst defences of the Premier League, but it does tell you some important things.
Firstly, it demonstrates the value of a consistent goalkeeper. Too often statistics just list the number of clean sheets won (the basis of the Golden Glove award) without taking into account number of shots faced, number of errors, etc.
It also gives you a snapshot of the most and least efficient defences in the league. The defence (including the goalkeeper) is responsible for stopping shots, and therefore stopping goals.
So it's interesting to note Wigan, Newcastle, Southampton and Aston Villa all find themselves in the top 5 worst in this category (QPR & Reading are sixth and seventh worst) - all have battled relegation this season.
In contrast, the top 5 best includes Chelsea and Manchester United, two clubs with keepers in excellent form, and Sunderland's Simon Mignolet and Swansea's Michel Vorm - two of the best outside the top four.
Now, there's a few surprises as well - Tottenham in the bottom five, Sunderland in the top for example. But lets take a closer look at one of the biggest. Manchester City, second in the Premier League, have one of the worst ratios - level with relegation strugglers Aston Villa.
But think about it. Manchester City have given up by far the fewest number of shots on target this season (98). That should make them one of the best defences - but it doesn't, not in isolation at least. They've also conceded the least number of goals (27) but relative to their league rivals that ratio (98 shots/27 conceded =3.6-shots-on-target-per-goal-conceded) is just not good enough.
If Manchester City had conceded at the same rate as their rivals Manchester United, they would have shipped just 20 league goals this term instead of 27. That's a lot of goals, and potentially a huge number of points.
The stat also passes the eye test - to any Premier League fan it's clear that City's defence is nowhere near as good as last season.
In fact, last year City (eventual Premier League champions) ranked towards the top with a ratio of 4.6-shots-per-goal-conceded - that would have put them tied-fourth for the best this season and level with the team likely to replace them as champions, Manchester United.
Vincent Kompany and Joleon Lescott had formed a solid partnership and Joe Hart was in excellent form.
This season the consistency is gone. Roberto Mancini flips between three at the back and four, Kompany has spent time on the sidelines, and Lescott has been benched. Joe Hart's dip in form is well documented, and individual mistakes have cost Mancini's side dearly. They've dropped off hugely in terms of defensive performance, and it's hurt their Premier League title challenge.
Of course, a huge factor has been City's lack of goals (Aguero's strike against United was just his 10th of the league campaign). But it's clear that their defence, once you scratch under the surface of the headline numbers, has actually hugely under-performed this season. Roberto Mancini may well blame his goal-shy strikers, but he should look to his back-line for answers as well.
Looking at the rest of the bottom five teams, it was always clear that Southampton's main problem was defence. Their early season form was awful (3 conceded against United & City, 6 against Arsenal, 3 versus Everton, 4 against West Ham).
Since the New Year they've only conceded more than two in the Premier League once - losing 4-2 to Newcastle in February. But the damage was done at the start.
Looking at the top clubs and it's no surprise to see Chelsea and Manchester United near the summit. Boasting Rio Ferdinand, Nemanja Vidic, David Luiz, Gary Cahill and John Terry, these two clubs have some of the league's elite defenders.
But lets look in more detail at the league's best - West Ham United.
Under Sam Allardyce they are famously difficult to play against and even more difficult to watch, but Big Sam's long-ball style is undeniably effective, and the stats prove it. He invites the opposition to monopolise the ball - West Ham have one of the lowest average possession percentages in the league at just 44 per cent. They don't care though, they're not Barcelona, they've got Andy Carroll!
But that gives their opponents ample opportunity to get shots on goal, and they've conceded the second highest number of shots-on-target in the league (221). Only Reading have allowed more efforts on target and they're bottom of the table! In theory then, that should make West Ham's defence terrible, and more likely to concede more goals. But it doesn't, and here's why.
The Hammers do an excellent job of frustrating opponents, and forcing them into 'low-percentage' shots - i.e. from outside the six-yard box.
The Hammers force their opponents to shoot from distance and they face the highest percentage of shots from outside the box (51 percent) - exactly the kind of efforts that are the most likely to be unsuccessful, and the easier to save.
To emphasise the point, their defensive style means they face the second lowest percentage of shots from inside the six yards box (5 per cent) - exactly the kind of efforts that are typically the most successful i.e. easy tap-ins and close-range finishes. They restrict 'high percentage' opportunites, making them the most efficient defence in the division.
So while they let opponents shoot, West Ham do so in such a way as to limit their opponents' chances of success. They let them rain in from long distance, thereby making it easier for Jääskeläinen to save them. It's the perfect combination defence.
And then Big Sam's 'play the percentages' style of football gets his team plenty of good chances at the other end of the pitch - 69 per cent of West Ham's shots come in the opponent's penalty box - by far the best in the league. Nice little tap-ins and flick-ons for Kevin Nolan et al to gobble up.
Allardyce clearly doesn't want his team taking the same type of 'low percentage' shots as his opponents. Because West Ham have the lowest percentage of shots from outside the penalty box in the Premier League, just 31 per cent.
That's the West Ham recipe for success. Force the opponent into long distance efforts once they've passed themselves to death, while you launch it forward quickly and get close range shots for yourself.
We've explored some of the reasons behind the statistics here, and we hope we've not bored you too much.
Take a look at the best and worst ratios below, and have your say on why you think certain teams make the top 5 or bottom 5.
Worst shots-per-goal-conceded ratio
Wigan | 2.95-shots-per-goal-conceded
Southampton | 3-shots-per-goal
Tottenham | 3.27-shots-per-goal
Newcastle | 3.28-shots-per-goal
Aston Villa, Manchester City | 3.6-shots-per-goal
Best shots-per-goal-conceded ratio
West Ham | 4.91-shots-per-goal-conceded
Swansea | 4.85-shots-per-goal
Chelsea | 4.7-shots-per-goal
Manchester United | 4.6-shots-per-goal
Sunderland | 4.3-shots-per-goal