Last year I read the book Poorly Made in China. At one point the author touches on this - he says that every time there's a census, the local authorities running it think "well, there's probably some people that we didn't catch, so let's add X percent to the total". Then those data sets get higher up, and these people think "well, there probably are some people those guys didn't catch, so let's add X percent to the total". That happens on a couple subsequent levels and in the end the population is overestimated by a large degree but noone has a way to know how much exactly.
Last year I read the book Poorly Made in China. At one point the author touches on this - he says that every time there's a census, the local authorities running it think "well, there's probably some people that we didn't catch, so let's add X percent to the total". Then those data sets get higher up, and these people think "well, there probably are some people those guys didn't catch, so let's add X percent to the total". That happens on a couple subsequent levels and in the end the population is overestimated by a large degree but noone has a way to know how much exactly.