IMF Working Paper
Oct 13, 2011
Summary: Current estimates of global poverty vary substantially across studies. In this paper we undertake a novel sensitivity analysis to highlight the importance of methodological choices in estimating global poverty. We measure global poverty using different data sources, parametric and nonparametric estimation methods, and multiple poverty lines. Our results indicate that estimates of global poverty vary significantly when they are based alternately on data from household surveys versus national accounts but are relatively consistent across different estimation methods. The decline in poverty over the past decade is found to be robust across methodological choices.
Introduction
Global poverty monitoring has been brought to the forefront of the international policy arena with the adoption of the Millennium Development Goals (MDG) by the United Nations. The first MDG proposes reducing global poverty by the year 2015 and is stated as “halving the proportion of people with an income level below $1/day between 1990 and 2015” (United Nations, 2000). Progress towards attaining this MDG is monitored using global poverty estimates published by the World Bank and a number of independent scholars. The process is not only expensive (Moss, 2010) but also mired with conceptual, methodological, and datarelated problems (Klasen, 2009).
Current estimates of global poverty proposed in the literature differ in magnitude as well as in the rate of change in poverty. Consider, for instance, Chen and Ravallion (2010) and Pinkovskiy and Sala-i-Martin (2009)—two studies that estimate global poverty using the international poverty line of $1/day (see Figure 1). Chen and Ravallion (2010) estimate that in 2005 nearly 26 percent of the population in the developing countries was poor, and the global poverty count fell by 520 million individuals since 1981. By contrast, Pinkovskiy and Sala-i-Martin (2009) estimate poverty to have been ten times lower in 2005, which implies a reduction of almost 350 million individuals since 1981. Although there is general agreement that global poverty has declined over the years, the estimated level of poverty and rate of poverty decline vary substantially across studies.
This paper aims to contribute to the debate on global poverty not by providing a new set of estimates, but by addressing two important questions. First, we ask why estimates from different studies differ so much. As we unravel the various assumptions made by researchers, we show that global poverty estimates are simply not comparable across studies. For instance, they differ in terms of underlying data sources, number of countries included, welfare metric, adjustments to mean incomes, and statistical methods employed to estimate the income distribution. Given this variety of methodological choices, we arrive at our second question: Can we assess the impact of different approaches on the resulting poverty estimates? Since global poverty estimation requires making multiple assumptions simultaneously, we aim to isolate and assess separately the relative importance of each such assumption by undertaking a novel sensitivity analysis.
An important hurdle in estimating long-term trends in global poverty is the lack of high-quality, consistent survey data. The poor are those individuals whose income is less than or equal to some threshold set by the poverty line. If countries had complete information on every individual’s income then with an agreed-upon global poverty line, identifying the poor would be a straightforward exercise. However, there are severe data limitations.
Data on income is typically collected through household surveys (HS) of nationally representative samples. However, survey data are often available for periods far apart and suffer from a number of inconsistencies (regarding sampling and interviewing techniques, definitions of variables, and coverage) that render them incomparable across countries. Nonetheless, they are the sole source of information on the relative distribution of incomes in a country—that is, the shares of national income possessed by different population groups (quintiles, deciles). HS also provide estimates of mean income/consumption which are used to scale the income shares to obtain mean incomes by population group. A more readilyaccessible and consistently-recorded source of information are national account statistics (NAS) which also provide aggregate income or consumption estimates and are available for most countries on a yearly basis.
A key methodological choice in estimating global poverty is whether to use data on mean income/consumption from HS or NAS or whether to combine data from the two sources. Some studies in the literature analyzed the sources of discrepancies between the levels and growth rates of income/consumption data from HS and NAS (Ravallion, 2003; Deaton, 2005). However these studies did not measure the precise effect of using HS and NAS data on global poverty levels and trends. In order to determine how sensitive global poverty estimates are to alternate data sources, we estimate global poverty by anchoring relative distributions alternately to HS and NAS estimates of mean income and consumption. This is our first sensitivity exercise.
The second sensitivity exercise concerns the choice of statistical method used to estimate income distributions from grouped data, that is, data on mean income or consumption for population groups (quintiles, deciles). We estimate global poverty by estimating each country’s distribution using different methods. These include the General Quadratic (GQ) and the Beta Lorenz curve, and the lognormal and Singh-Maddala functional forms for the income density function.2 In addition to these parametric specifications, we also consider the nonparametric kernel density method whose performance we assess in conjunction with four different bandwidths—a parameter that controls the smoothness of the income distribution.
As a benchmark, we follow the World Bank methodology to the extent possible and estimate global poverty in 1995 and 2005—the latest year for which data is available for many countries. Data on the relative distribution of income across population deciles is collected for 65 countries from the World Bank’s poverty monitoring website PovcalNet. Our sample covers more than 70 percent of the total world population and includes all countries for which both HS and NAS data are available in both years. Global poverty is estimated using international poverty lines ranging from $1/day to $2.5/day to provide further insight into how methodological choices impact poverty rates at different income cutoffs.
Our results are twofold. First, a large share of the variation in estimated poverty levels and trends can be attributed to the choice between HS and NAS as the source of data. Global poverty estimates vary not only in terms of the proportion of the poor, and correspondingly the number of poor, but also in terms of the rates of decline in poverty. Poverty estimates based on HS and NAS do not tend to converge in higher income countries. Second, the choice of statistical method used to estimate the income distribution affects poverty levels to a lesser extent. A comparison of poverty estimates across parametric and nonparametric techniques reveals that the commonly used lognormal specification consistently underestimates poverty levels. While there is little doubt that the proportion of poor declined between 1995 and 2005, our results underscore the fact that global poverty counts are highly sensitive to methodological approach.
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