quintiles, social mobility, household income
Movement across quintiles of household income has become a standard measure of social mobility. This choice of what to count (households rather than people) is consequential. Earlier, absolute measures of social mobility (such as the percentage of sons of blue-collar fathers gaining white-collar positions). However, measuring movement across quintiles conceptualizes social mobility as relative, as a zero-sum game (in that it seems obvious that for each person who moves to a higher quintile, another must fall). However, there are problems with using quintiles. First, each quintile contains a fifth of all households, rather than a fifth of all people. There are far more people in the top quintile than in the bottom. Second, while income inequality is increasing, the increase is not because the proportion of the population that is low-income is increasing, but because the upper-middle class is expanding (and now includes more than a fifth of all households). While choosing to measure mobility in terms of quintiles is not mathematically wrong, it shapes the way the data can be read. Understanding the consequences of such choices is relevant to understanding the numeracy of the measure.
Best, Joel. "Questioning Quintiles: Implications of Choices of Measures for Income Inequality and Social Mobility." Numeracy 11, Iss. 2 (2018): Article 6. DOI: https://doi.org/10.5038/1936-46184.108.40.206
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