The Daily Telegraph published a story today claiming that Theresa May could end freedom of movement at some point next month.

To illustrate this article they used a map showing the impact of immigration in English and Welsh local authorities.

It looks like this:

Because I’m a data geek, I always pay a lot of attention to maps, but this one raised many questions.

1. What data is it visualising?

At first, I thought the map was showing the percentage of foreign-born population in each local authority. This is the kind of data used for maps made by the ONS or the Migration Observatory at Oxford University, but something didn’t feel right this time.

In 2015, the UK population was 13.5% foreign-born, but according to the Telegraph’s map there are dozens of local authorities where migrant population is around 50%.

This can’t be right.

2. What do they mean by migration rates?

When we talk about ‘net migration rate’ we mean the difference between immigrants and emigrants of an area in a period of time.

This is a very common metric, but on this map there isn’t any reference to a time period.

3. Why do they use Home Office data?

Working with immigration data can be tricky, particularly when it’s about migration within the EU.

The most common source used to show the percentage of foreign population comes from the Office for National Statistics but the Telegraph quotes the Home Office. I’m not sure which dataset they used because there’s no link.

From what I’ve seen on Twitter, many people understood that this map visualises the percentage of population who are migrants, and they pointed out that it is showing a very different picture compared to the ONS data.

You can see the difference looking at these 2 maps