2010年11月17日星期三

收入差距及变动

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。

这出自《经济研究》上的一篇文章《中国居民家庭的收入变动及其对长期平等的影响》。(很奇怪,面板数据被翻译成“模块数据”。)其实弗里德曼早就说过:

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.

再来一篇Evolution of Income Mobility in the People’s Repubilc of China: 1991–2002的部分摘要:

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.

后面的讨论都按此分类。

是想找张维迎的一项研究,才牵扯出上面这些。他在改革三十年的时候说,看中国的跨地区数据,以基尼系数衡量收入差距:

  1. 收入水平越高的地方,平均而言收入差距反倒越小,如果我们按照增长率来看,也是一样的,平均而言,增长率高的地区它的收入分配的差距反倒比较小,特别像浙江、福建、广东这些地方,而收入增长比较低的地区,恰恰收入分配的差距反倒比较大;
  2. 国有部门占比例越高的这些地区,收入的差距反倒越大,而不是越小;
  3. 政府花钱GDP比重越高的地方,基尼系数越大,也就是收入分配的差距越大;
  4. 利润占GDP当中比重越高的地方,它的收入差距反倒越小,而利润占GDP比重越小的地方,收入差距越大;
  5. 市场化程度越高的地区,收入差距反倒越小,而不是越大。

我始终没有找到他这项研究相应的论文。莫非不是挂他的名字发表的?读者有知道的吗?

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