The article suggests a way to estimate how people's incomes change over time using only general income data. By looking at the average and spread of incomes, we can figure out the range of income mobility. The study found that traditional ways of measuring income growth for different groups can be misleading if they don't consider mobility. The research used data from the U.S. and many other countries to show that the income growth of the bottom 40% and top 10% is often underestimated or overestimated when mobility is not taken into account.