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Hans Rosling: As a statistician the most well used item in my toolkit is the humble average. It’s the simplest but most powerful way to get a ready handle on a mass of confusing data.

What’s so great about the average is that you can take a whole mass of data and reduce it to a single number. And although each of us is unique, our collective lives produce averages that can characterise whole populations.

Michael Marmot: I looked in my local newspaper one week and saw a pensioner had accidentally put her foot on the accelerator and crushed her friend against a wall - devastating, hideous, horrible thing to happen. And then there was the second one about a young man who didn’t have a driving licence, was driving a car under the influence of drugs and alcohol, and he bashed into a pedestrian and killed him.

What’s remarkable, absolutely remarkable, if you look at the number of people who die each year in traffic crashes, it’s nearly a constant. What? Of all these individual events, somehow, when you sum them all up, there’s the same number every year. And every year two and a half times as many men die in traffic crashes as women and is a constant. And every year the rate in Belgium is double the rate in England. There are these remarkable regularities so that these individual particular events sum up into a social phenomenon.

Hans Rosling: ‘Let’s see what Sweden have done; they used to boast about fast social progress. That’s where we were…’

In my lectures to tell stories about the changing world I use the averages from entire countries, whether the average of income, child mortality, family size or carbon output.

‘Okay, I’ll give you Singapore. The year I was born, Singapore had twice the child mortality of Sweden, the most tropical country in the world, a marshland on the Equator and here we go. It took a little time for them to get independent but then they started to grow their economy and they made a social investment, they’ve got away malaria, they’ve got a magnificent health system that beat both the US and Sweden. We never thought it would happen that they would win over Sweden!’ [Applause]

But useful as averages are, they don’t tell you the whole story.

On average, Swedish people have slightly less than two legs. This is because a few people only have one leg or no legs and no one has three legs, so almost everybody in Sweden has more than the average number of legs. The variation in data is just as important as the average.

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Hans Rosling talk at TED