As far as I can tell, his study is looking for a correlation with the distance to Metro stations.
This is a big difference. There are hundreds of Metro stations in Paris. Everywhere is close to one.
I think the original intent was distance to a train station. If Paris is anything like Rome, close to the railway station is cheap hostels and recent immigrants accommodations.
The original data includes "train and metro stations", but figure 9 filtered the data to only include train stations and arrived at the same conclusion.
That saying in France is usually understood to be for cities outside of Paris and only referring to "Gares" (that word is used for train stations, not for subway stations). Anecdotally, I'd say it holds true in general in most cities I've visited (with Paris being an exception)
This is a big difference. There are hundreds of Metro stations in Paris. Everywhere is close to one.
I think the original intent was distance to a train station. If Paris is anything like Rome, close to the railway station is cheap hostels and recent immigrants accommodations.