> Random would mean 20% of ending in the same quintile, perfect predictor would mean 100%, so 40% is much closer to random. Something that doubles or reduces

> Random would mean 20% of ending in the same quintile, perfect predictor would mean 100%, so 40% is much closer to random. Something that doubles or reduces by 10x the random probability is not "closer to random". I doubt that if you tell something that a factor is "closer to random than to a perfect predictor" they'd expect something like that. > You say "should" but it seems like this is only true if you expect (and want) completely random results. There is quite a lot of randomness involved in where one ends up. There is a random distribution of ability, of work industry, location, country, luck, connections, timings... quite a lot of factors so that you'd expect that randomness to dominate over the whole population and things be close to ~20%. In fact, countries with higher social mobility tend to be fairly close to that 20%. In Spain (which is by no means the best at social mobility, but it's the only one I have data at hand from) we are pretty far from that 43% chance of staying at the lower quintile, and also far from the 3% of moving from bottom to top: https://widgets.elpais.com/mapbox/2020/07/atlas-renta/pc_nacional_edad/pc_nacional_edad-tableta.jpg?v=3398 https://widgets.elpais.com/mapbox/2020/07/atlas-renta/pc_nac...