Thomas Garrett of St Louis Fed serves a nice reminder on limitations of calculating inequality.
Income quintiles from different years are often compared to demonstrate the growing income inequality over time between, say, the poorest 20 percent of households and the wealthiest 5 percent of households. For example, in 1970 the income of the wealthiest 5 percent of households was 6.3 times greater than the income of the poorest 20 percent of households, whereas in 2007 the income of the wealthiest 5 percent of households was 8.8 times greater than the income of the poorest 20 percent of households. When such comparisons are made, it is implicitly assumed that each quintile contains the same households over time.
For most people, income increases over time as they move from a usually low-paying first job to better-paying jobs later in life. Some others, however, may lose income over time due to business cycle contractions, demotions, career changes, retirement, and so on. Because incomes are not constant over time, the same households do not necessarily remain in the same income quintiles. Thus, comparing income quintiles from different years is a proverbial apples-to-oranges comparison because the households compared are at different stages in their earnings profile.
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Mostly Economics 
