From a 2010 St. Louis Fed report "U.S. Income Inequality: It’s Not So Bad
"One big problem with inferring income inequality from the census income statistics is that the census statistics provide only a snapshot of income distribution in the U.S., at a single point in time. The statistics do not reflect the reality that income for many households changes over time—i.e., incomes are mobile. For most people, income increases over time as they move from their first, low-paying job in high school to a better-paying job later in their lives. Also, some people lose income over time because of business-cycle contractions, demotions, career changes, retirement, etc. The implication of changing individual incomes is that individual households do not remain in the same income quintiles over time. Thus, comparing different income quintiles over time is like comparing apples to oranges, because it means comparing incomes of different people at different stages in their earnings profile.
The U.S. Treasury released a study in November 2007 that examined income mobility in the U.S. from 1996 to 2005. Using data from individual tax returns, the study documented the movement of households along the distribution of real income over the 10-year period. The study found that nearly 58 percent of the households that were in the lowest income quintile (the lowest 20 percent) in 1996 moved to a higher income quintile by 2005 (see top chart above). Similarly, nearly 50 percent of the households in the second-lowest quintile in 1996 moved to a higher income quintile by 2005. Even a significant number of households in the third- and fourth-lowest income quintiles in 1996 moved to a higher quintile in 2005.
The Treasury study also documented falls in household income between 1996 and 2005. This is most interesting when considering the richest households. As shown in the bottom chart above, more than 57 percent of the richest 1 percent of households in 1996 fell out of that category by 2005. Similarly, more than 45 percent of the households that ranked in the top 5 percent of income in 1996 fell out of that category by 2005.
Thus it is clear that over time, a significant number of households move to higher positions along the income distribution, and a significant number move to lower positions along the income distribution. Common reference to “classes” of people (e.g., the lowest 20 percent or the richest 10 percent) is quite misleading because income classes do not contain the same households and people over time."
sums up the issue of income inequality and income mobility this way:
"Only by focusing on the income brackets, instead of the actual people moving between those brackets, have the intelligentsia been able to verbally create a "problem" for which a "solution" is necessary. They have created a powerful vision of "classes" with "disparities" and "inequities" in income, caused by "barriers" created by "society." But the routine rise of millions of people out of the lowest quintile over time makes a mockery of the "barriers" assumed by many, if not most, of the intelligentsia."