I seem to be in a conservative mood.
Mobility is often measured by dividing the population into fifths by earnings, then determining the number of people moving from one fifth to another. For example, probably all of the first round picks in the recent NFL draft are moving into the top fifth of earnings from a lower fifth, perhaps in many cases the lowest fifth.
I don't know about the rest of the world, but when I read about "fifths" I don't think about differences among the people comprising the "fifth", I think about a stereotype: "top fifth" would be a lawyer or financial type; middle fifth would be a white collar worker, bottom fifth would be manual laborer. That's not quite right, but I hope it conveys my idea: I'm imagining a lot of people with the same characteristics. In reality, of course, I should be thinking about pro athletes and entertainers and business owners being in the top fifth.
And in the bottom fifth, I should be thinking about the people within the correctional system, the people on SSDI because of physical or mental disability, the illiterate, and so forth. In other words, when we talk about the possibility of people moving up from the bottom fifth, there's a good proportion, perhaps 40 percent, for whom a miracle must happen to be able to move up.