[...] there is both continuity and change in people's circumstances from year to year. We do not live in a country where there is an annual lottery to determine at random who is rich and who is poor for the coming year regardless of where they started last year. But nor do people generally stay stuck in the same place. Those who start poor are more likely to stay poor the next year than those who do not, and those who start rich more likely to stay rich. But many in each group--and those in between--move up or down the income ladder each year.