[redwood]

Right: how do they calculate the <$1/day rate. Its nearly inverse relationship with the GDP per capita graph implies that deep down in the data the basis for this $1/day rate could essentially be the GDP rate divided by population, or similar. I.e. without seeing where that data comes from this is a real possibility. However I would assume these folks are doing a rigorous analysis so this is probably unlikely.

[Kliment]

Since they point out this inverse relationship as remarkable, I don't suppose there is an expected relationship of this sort between the two. Certainly there would be a negative correlation, but I don't believe they are coming from the same data.