Mapping London's different densities

8 Feb 2014 21:03 GMT

This week, Londoners were confronted with the challenge of trying to get to and from work with most of the Tube network either suspended or running a reduced service for two days. Whatever your opinion of the strike, it's hard to deny that the impact of closing the Tube is large. While playing around with some population data today, I discovered one of the reasons why.

One of the ways you can quantify how London's population changes during the day is to compare the residential population of London with the workday population. The residential population is based on where people live — it's the conventional measure of the population that is used in surveys and the Census to allocate each person living in the country to the population of a particular area given their home address.

But many people spend most of their waking life outside their home, at their place of work. The workday population is an alternative measure of the population, which is based on where working people work and non-working people live — it's a way of allocating people to the population of an area based on where they spend their working lives if they work, or where they live if they don't have a usual place of work or don't go out to work.

The difference between the residential population and the workday population does not provide a direct measure of the movement of people in and out of London during the day. First of all, the difference between the two populations is the net difference between people moving in and out of London when they go to work; and while most people travel into London to work, some people who live in central London travel out of London to work elsewhere. Second, the workday population includes shift and night workers, who are not at work during the daytime. Third, neither measure of the population includes short-term residents or visitors to London.

Nevertheless, comparing the two populations shows that the workday population is significantly larger than the residential population in the most central parts of London, and the effect is concentrated in a very small area.

This visualisation shows the population density of Parliamentary constituencies in London, based on both their residential and workday populations, as recorded at the 2011 Census. The map alternates between the two measures, and is shaded to represent the density of both populations using the same linear scale, where white is the lowest recorded density in any constituency, and black is the highest.

The map shows that a small number of constituencies in central London have much larger workday populations than residential populations. Based on the 2011 Census, the workday population of the Cities of London and Westminster constituency is around 946,000, which is almost nine times the residential population of 110,000. And because the constituency covers an area of just 16.4 square kilometres, the density of the workday population is around 57,700 people per square kilometre.

Other constituencies with large differences between their workday and residential populations are Holborn and St Pancras (+174,000), Poplar and Limehouse (+97,000), and Islington and Finsbury (+73,000). But there is no other constituency in London whose workday population density is comparable to the Cities of London and Westminster. The constituency population with the next highest density is the workday population of Holborn and St Pancras, which at 23,500 people per square kilometre is less than half as dense.

With so many people trying to get into such a small area at the very heart of London, it's no wonder a Tube strike can be so disruptive.

The FT makes a statistical mistake

16 Feb 2014 10:32 GMT

Update, 28 Feb 2014: Since publishing this post, and thanks to an intervention by the FT's stats and data team, the paper has now corrected the error discussed below.

Last week, the FT published an article suggesting that the number of British nationals living in other EU countries was roughly equal to the number of other EU nationals living in the UK. The story was based on two sets of figures:

1. Estimates of the number of British nationals living in other EU countries published by the Institute for Public Policy Research (IPPR) in 2010, which were recently cited in a response to a Parliamentary Question. These figures suggest that there are around 2.2 million British nationals living in other EU countries in total.

2. Office for National Statistics (ONS) estimates of the UK population by nationality. These figures show that there are around 2.34 million nationals of other EU countries living in the UK.1

However, these two figures are not comparable because the IPPR figure includes people living abroad for only a part of the year, while the ONS population estimates do not include such people, and only count people usually resident in the country for 12 months or more.

This makes a significant difference to the comparison, as there are 410,000 British nationals included in the IPPR figure that are only living abroad for a part of the year. When these people are removed and the figures are compared on the same basis, the IPPR figure is more than half a million (555,000) or 24% smaller than the ONS figure.2 The two figures are therefore not equal and the premise of the story is false.

I tried contacting the FT both through Twitter and by sending an email to their corrections department. The paper has said it is not going to correct the story, on the grounds that it was simply reporting two official figures in good faith.

This sidesteps the question of whether the ONS figure really does exclude people living in the UK for only a part of the year (it does — see page 4 of this, or page 1 of this, or section 1 of this for example); and it overlooks the fact that, while the Goverment does seem to have signed up to the IPPR estimates for now, it never said they were comparable with ONS data. The FT said that.

This is a big disappointment to say the least. Partly because it is unusual for the FT to overlook such a mistake once it has been pointed out. But more because of the paper's lack of intellectual curiosity on this occassion. Because here's the thing: no-one knows with any certainty how many British nationals are currently living in the EU.

The European Union's official statistics agency Eurostat publishes estimates of the number of British nationals living in each EU country; and the sum of the most recent available estimates suggests that there are around 870,000 British nationals living in other EU countries. Data is missing for some countries, and data for other countries is old, so 870,000 is probably an underestimate; but we don't know by how much.

Because of gaps like this in the data, when the IPPR did their study — which estimated the number of British nationals living abroad in 2008 — they applied various upward adjustments to the official figures to compensate for the possibility of undercounting. But we have no way of testing the validity of these adjustments, because we would need to know the actual number of British nationals living in each country to see if they were justified, and if we knew that we wouldn't have to guess. We simply don't know how accurate the IPPR's adjustments are because (unlike the ONS figures) their figures are not the product of a statistical method that allows you to quantify the degree of uncertainty surrounding each estimate. There are no confidence intervals here.

It would have been great to see the FT explaining all of this, and setting out the considerable uncertainty surrounding this known unknown; instead of pretending to know something it doesn't.

Footnotes

1. They're in Table 2.1 of the reference tables if you can't find them.

2. If you want to see the actual numbers involved in this calculation you can download a spreadsheet of the relevant IPPR figures here. They are taken from Appendix C of the IPPR Report.