we're worse off than those in Egypt, that many don't grasp the Gini index, and that there are all sorts of problems with it. That this has been posted a couple of dozen times.
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o Transfer principle: if income (less than half of the difference), is transferred from a rich person to a poor person the resulting distribution is more equal.
Disadvantages of Gini coefficient as a measure of inequality
* While the Gini coefficient measures inequality of income, it does not measure inequality of opportunity. For example, some countries may have a social class structure that may present barriers to upward mobility; this is not reflected in their Gini coefficients.
* If two countries have the same Gini coefficient but one is rich and the other is poor, it can be seen to measure two different things. In a poor country it measures the inequality in material life quality while in a rich country it measures the distribution of luxury beyond the basic necessities.
* The Gini coefficient of different sets of people cannot be averaged to obtain the Gini coefficient of all the people in the sets: if a Gini coefficient were to be calculated for each person it would always be zero. For a large, economically diverse country, a much higher coefficient will be calculated for the country as a whole than will be calculated for each of its regions. (The coefficient is usually applied to measurable nominal income rather than local purchasing power, tending to increase the calculated coefficient across larger areas.)
* The Lorenz curve may understate the actual amount of inequality if richer households are able to use income more efficiently than lower income households or vice versa. From another point of view, measured inequality may be the result of more or less efficient use of household incomes.
* Economies with similar incomes and Gini coefficients can still have very different income distributions. (This is true for any single measure of a distribution.) This is because the Lorenz curves can have different shapes and yet still yield the same Gini coefficient. For example, consider a society where half of individuals had no income and the other half shared all the income equally (i.e. whose Lorenz curve is linear from (0,0) to (0.5,0) and then linear to (1,1)). As is easily calculated, this society has Gini coefficient 0.5 -- the same as that of a society in which 75% of people equally shared 25% of income while the remaining 25% equally shared 75% (i.e. whose Lorenz curve is linear from (0,0) to (0.75,0.25) and then linear to (1,1)).
* It measures current income rather than lifetime income. A society in which everyone earned the same over a lifetime would appear unequal because of people at different stages in their life. However, Gini coefficient can also be calculated for any kind of single-variable distribution, e.g. for wealth.<13>
* Gini coefficients do include investment income; however, the Gini coefficient based on net income does not accurately reflect differences in wealth—a possible source of misinterpretation. For example, Sweden has a low Gini coefficient for income distribution but a significantly higher Gini coefficient for wealth (for instance 77% of the share value owned by households is held by just 5% of Swedish shareholding households).<14> In other words, the Gini income coefficient should not be interpreted as measuring effective egalitarianism.
* Too often only the Gini coefficient is quoted without describing the proportions of the quantiles used for measurement. As with other inequality coefficients, the Gini coefficient is influenced by the granularity of the measurements. For example, five 20% quantiles (low granularity) will usually yield a lower Gini coefficient than twenty 5% quantiles (high granularity) taken from the same distribution. This is an often encountered problem with measurements.
* Care should be taken in using the Gini coefficient as a measure of egalitarianism, as it is properly a measure of income dispersion. For example, if two equally egalitarian countries pursue different immigration policies, the country accepting higher proportion of low-income or impoverished migrants will be assessed as less equal (gain a higher Gini coefficient).
* The Gini coefficient is a point-estimate of equality at a certain time, hence it ignores life-span changes in income. Typically, increases in the proportion of young or old members of a society will drive apparent changes in equality. Because of this, factors such as age distribution within a population and mobility within income classes can create the appearance of differential equality when none exist taking into account demographic effects. Thus a given economy may have a higher Gini coefficient at any one point in time compared to another, while the Gini coefficient calculated over individuals' lifetime income is actually lower than the apparently more equal (at a given point in time) economy's.<15> Essentially, what matters is not just inequality in any particular year, but the composition of the distribution over time.
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http://en.wikipedia.org/wiki/Gini_index#Disadvantages_of_Gini_coefficient_as_a_measure_of_inequality