Dorling - The 32 stops: lives on Londons Central line

And it is not so much the exercise people take, or how healthily they eat, or whether or not they smoke that matters. Its much more who ends up living here. At the very start of the line those who do better in life end up living, on average, further out from the centre. But luck has a lot to do with where you end up living along the line too, good luck as well as bad luck.

The line is over a century old. Its westernmost station was opened on 2 April 1911, on the very same Sunday that the groundbreaking 1911 Population Census was taken, the first census to make a detailed record of living conditions, main employer and the number of families living in each home. It recorded the industry or services with which the workers of the households were connected, how long couples had been married and how many children were born alive, how many were still alive and how many had died. It was the census from which our current coding of social class was first derived and through which we first recorded our intention to improve infant health.

Everything you are about to read is based on fact. These facts are taken from official records. They come from childrens GCSE school exam certificates, from pensioners death certificates recording the length of life of each and assigning each an underlying cause of mortality. These facts come from censuses and surveys, counts of bankers and tax office estimates of average incomes. They also come from others descriptions, such as the official summaries of the number of children growing up in poverty in each place. What all these facts do is determine the social gradient between places, in this case between tube stops of the Central Line. They can tell us at which point, and in which aspects of life, we are heading up or down, socially, as we travel west to east, geographically.

Exactly a century after the 1911 census was taken, and after the furthest tube line west was opened, a couple are arguing in bed. They are arguing about the census and why they chose to live in West Ruislip.

Dont forget the baby, he said.

How could she forget the baby? Theyd moved for the baby, for the extra room, for the future. The baby was much more important than the census form. The form that had been sitting by the kitchen sink for a week.

Someone at work was telling me, only yesterday, that people always forget to add the baby, he said.

He worked in town, in Islington, he changed at Bank, but in May the office was moving to Pimlico and then hed change at Oxford Circus.

Thats why we didnt choose to live there, he said.

He liked statistics. He worked for the Office for National Statistics. It took him exactly 35 minutes to do most of the Daily Telegraph crossword, leaving precisely a minute to sit down in his seat and precisely a minute to get up.

Babies do not work to timetables. Babies do not derive pleasure from being told that their parents have mortgaged themselves beyond the hilt to secure them a premier postcode, a place where the average GCSE score is 356 points. He had annoyed her intensely when he decided he needed to explain that particular school statistic to her as they were looking at possible semis to buy. Again his colleague at work had explained it to him.

The average child in London is awarded 337 points for its GCSEs, but the average for West Ruislip is 356 points. Although our child wont be average; our child will be very clever.

He was going to go on to try to explain how GCSE result scores are calculated, but he had lost her attention.

Babies get left off census forms all the time, he had said, just last night, as she was falling asleep. Her nightmare had been about forgetting the baby, not being able to add up the numbers, finding she was living in a place because of the numbers, because of the numbers about the neighbours.

And its 90 per cent white, he told her.

We shouldnt be choosing where to live because of that, she said.

No, he agreed.

Exactly three minutes down the line is Ruislip Gardens, GCSE point average of 353.

Almost slipping by a whole grade in one subject, he told her, as she rolled her eyes.

I dont care about the schools, she said. I dont care about the crime. I dont care about the numbers. I just want to get one decent nights sleep. But she did care, not as much as him, but enough to agree with him about where to live.

The thing about three-year-olds is that they think its waking-up time when its light, she tried to explain.