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 Originally Posted by BananaStand
that is NOT what I asked.
You yourself said that "one person can go up 3 quintiles, and 3 people can go down 1 quintile". right??
So my totally valid, cogent, and compelling question here is.....how likely are people to move up or down? In your hypothetical example, you say that people are 3x more likely to move down. It would be really interesting to know the real numbers on that, and the Gatsby chart doesn't show that information.
It's irrelevant to any overall conclusion based on the graph because the overall net upwards/downwards movement has to balance out.
If, say, all cases of upward mobility in country A were due to people going up one rung, and all cases of downward mobility were due to people going down two rungs, then twice as many people are going up as going down, but they're only going up 1 notch each while the half as many people who are going down are going down 2 notches each.
The only way such a situation would change the overall numbers is if income mobility in one country tended to involve larger leaps up or down, but that would make the index of mobility greater, not smaller, which it should.
So, e.g., the rags to riches story of someone going from Q5 to Q1 involves a change of four quintiles, which would have to be balanced by some combination of people going down by four quintiles; generally this would involve multiple people (although it could involve a single person going from Q1 to Q5, it doesn't require it). Conversely, a one-quintile change upward can only be balanced out by a one-quintile change downwards, which limits the number of people needed to balance the equation to 1.
So, the mobility index to some extent measures both the number of people who are mobile and the extent to which they are mobile. But these effects are additive and both contribute to the index, which they should. Parsing out the number and the extent values might be interesting, but it shouldn't change the overall shape of the graphs by much if anything. The basic moral is that no, the US doesn't suck on that graph because the index is penalising countries with large numbers of rags-to-riches stories. In fact, it's doing the opposite.
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