"Over time a significant number of households move to higher or lower positions along the income distribution. As a result, public policies such as income taxation and income redistribution affect ‘classes' of people differently over each person's lifetime. In addition, income mobility muddies the picture of income inequality derived from a simple comparison of income quintiles from different years because such comparisons incorrectly implicitly assume that each income quintile contains the same households over time."  St. Louis Federal Reserve.