The primary issue here is how you can define "wealth" or "rich" and "poor." Income's one measure, sure. But what about the trust fund kids? What about college kids?

The real problem with collecting data like this is that there's no real good source of it. Most of what you get has selection bias, and the other stuff you can get is riddled with holes. And then you try to do it across countries, where definitions change and governments college disparate data at differing granularities and it just gets...frustrating.

An economist's toughest job is finding good data. The analysis is the easy part!