人们常常用基尼系数来衡量一个地区居民收入的不平等。然而，基于横截面数据的指标，如基尼系数并不能反映长期内的实际不平等，而且有可能引起误导。如在有两个人，i和j的经济单位，以X1=(1i，0j)表示两个人在第一年的收入分配，以X2=(0i，1j)表示第二年的分配。这样，每一年的基尼系数都是1.00，收入分配极端不平等。但实际上在两年内，每人所得均为1，是一个极端平等的社会。之所以会这样，是因为基尼系数的计算依据静态的横截面数据，它不管给定的个体在不同时期的收入变化。因此为了更完整地反映收入分配不平等的状况，需要利用模块数据（panel data）来计算收入的变动（income mobility）。对于贫困的理解也存在同样的问题，仅仅知道处于贫困线以下人口和家庭的比例是不够的，我们还需要清楚在一给定的年份，一个贫困家庭会有多大的比例在下一年仍然陷于贫困、或脱贫。
Consider two societies that have the same distribution of annual income. In one there is great mobility and change so that the position of particular families in the income hierarchy varies widely from year to year. In the other, there is great rigidity so that each family stays in the same position year after year. Clearly, in any meaningful sense, the second would be the more unequal society. The one kind of inequality is a sign of dynamic change, social mobility, equality of opportunity; the other, of a status society.
Annual income data may provide a misleading indicator of enduring income inequality in societies where there is considerable year-to-year income mobility. Using two rounds of data on households, the paper measures income mobility in the People’s Republic of China (PRC) between the early 1990s and early 2000s. In the early 1990s, the increase in income inequality in the PRC was accompanied by a level of income mobility comparable to other developing countries in transition, and was higher than that found in developed countries such as the United States. By the early 2000s, however, while the PRC’s income inequality increased further, income mobility decreased, implying that the probability of being stuck in a relatively lower level of income increased for households.
关于这个问题，又看到一篇名为Income Mobility in China: Main Questions, Existing Evidence, and Proposed Studies的综述，挺不错。注意这一段：
It is important to recognize that there is not one aspect of macro mobility; rather, there are six different concepts, each of which can be measured using alternative indices (Fields, 2007). Time-independence measures gauge how dependent current income is on past income. On the other hand, movement measures address a different issue: in comparing some aspect of the same individuals’ incomes between one year and another, how much income movement has taken place? The various movement indices in the literature may usefully be categorized into five groupings or concepts ('concepts' because they are different underlying entities, not alternative measures of the same underlying entity). Positional movement (or 'quantile movement') is about the movement of individuals among various positions (quintiles, deciles, or centiles) in the income distribution. Like positional movement, share movement is relative but in a different way. Share movement takes place if and only if an individual's income rises or falls relative to the mean. Another concept is non-directional income movement (also called 'flux'), which gauges the extent of fluctuation in individuals' incomes. Income changes are measured but the direction of change (positive or negative) is ignored. When the direction of change is considered, we have the concept of directional income movement. Finally, income mobility as an equalizer of longer-term incomes compares the inequality of recipients’ incomes over a number of periods with the inequality of incomes measured at a single point in time.