There has been much discussion around the World Bank on the choice of a "global poverty target" that can be used to measure global progress against poverty. To be successful, such a target needs to be (a) simple to understand, and (b) relevant to all World Bank client countries.
poverty and inequality measurement
This follows on part I posted earlier today – we choose to split the post into two because of length.
Part 2: What does Martin’s proposal imply?
Survey responses to questions on incomes (and other potentially sensitive topics) are likely to contain errors, which could go in either direction and be found at all levels of income. There is probably also non-random selection in terms of who agrees to be interviewed, implying that we get the weights wrong too (as used to "gross-up" the sample estimates to the population).