population. Approximately 15,038 household members in the baseline were re-interviewed in the wave of 2001, and 12,100 of these were surveyed in 2006. These figures imply an approximately 20 percent attrition rate between 2001 and 2006, excluding all new households that branched out from the 2001 original ones. For Mexico, the data is taken from the _Mexican Family Life Survey_ (MxFLS) for the first (2002) and second (2005) rounds. The survey is representative at the national, regional, urban and rural levels. The first wave includes approximately 8,440 households distributed in 150 localities, and the second wave includes 7,572 of the original households for an attrition rate of only 10 percent. As before, we only consider those households interviewed in both 2002 and 2005, excluding all new households that branched out.

Finally, for Peru we use the panel of the _National Household Survey_ (ENAHO Panel) collected by the Institute of Statistics (INEI), providing a five-wave unbalanced panel for the period 2002-2006. The size of the sample (6,257 in 2002; 4,217 in 2003; 6,490 in 2004; 6,778 in 2005; and 6,593 in 2006) is representative at the national, urban, rural, and regional levels (Coast, _Sierra_ and _Selva_ regions). The unbalanced nature of the panel results in gaps in the observations, since there is no information for all five waves for every household. Therefore, in this paper we use the information of those households interviewed in both 2002 and 2006 (5,092 households), which imply an attrition rate of approximately 20 percent similar to that found in the case of Chile, and comparable to longitudinal surveys in other regions of the world.

Then, the definition of the middle class with the vulnerability-to-poverty approach is applied to obtain the lower PPP income threshold to cross-sectional data in order to analyze the evolution of the middle class over a longer period of time. For Chile (1992-2009), the data is taken from the cross-sectional CASEN, carried out every two years since 1985 by MIDEPLAN. This data source is a nationally and regionally representative household survey used to measure the incidence of poverty and the level of inequality in the country. For Mexico (1992-2008), we use the _National Household Consumption and Expenditure Survey_ (ENIGH), undertaken by the Institute of Statistics and Geography (INEGI). The ENIGH is a nationally representative survey, covering urban and rural areas, and it contains detailed information on income and different categories of expenditures. This survey is available for 1984 and 1989, and for every two years since 1992. Finally, in the case of Peru (1997-2009) we use data from the cross-sectional ENAHO, collected on a quarterly basis by INEI since 1997. The fourth wave of the survey is nationally representative, and it is used for poverty measurement as it collects information on income and expenditure. All three surveys collect information of dwelling characteristics, and indicators related to age, sex, employment, and education of the household members.

# **5. A vulnerability approach to middle class analysis**

We propose an empirical framework to define the middle class in terms of households‘ vulnerability to poverty following a regression-based approach to estimate an income lower threshold associated to a low probability of falling into poverty, which will be used for cross-

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