The PME sample of households was designed to be representative of the urban population for each of the six metropolitan regions covered by the survey. The sampling scheme adopted by the PME is characterised as a stratified two stage cluster design with unequal probabilities of selection. Samples of households are selected separately from each metropolitan region.
In each of the metropolitan regions groups of municipalities or pseudo- municipalities2 form the independent strata. The census enumeration sectors,
which are the primary sampling units (PSU), are selected independently from each stratum in each of the metropolitan regions. The secondary sampling units (SSU) are the households which are selected from each PSU. All residents from the selected households are surveyed. The motivation for this design is to ensure the spread of the sample within each metropolitan region.
The PSUs are selected by systematic sampling with probability proportional to their total number of private occupied households, as listed in the 2000 Demo- graphic Census. From the selected PSUs, the households are selected by simple systematic sampling, using a random start and a fixed interval of selection with
2The definition of pseudo-municipality is that when more than one municipality of smaller size, according to the 2000 Census, are joined together to represent one stratum with sufficient size to allow for the minimum number of PSUs to be selected from it (IBGE, 2002).
the initial goal of selecting 16 households per PSU. Each PSU remains in the sam- ple for a decade. It will only be replaced earlier by a similar one if there exists a shortage of households to be selected or if the whole selection process is renewed. Table 3.1 presents the PME sample composition. It shows the total number of selected PSUs and selected households when the sample for the last revision was designed, March 2001. The table also shows the equivalent information for December 2005, for comparison. The change over time is due to the rotation of the sample and the re-listing process carried out every year. Information for 2005 in this table was constructed from the monthly micro-data.
Table 3.1: The PME Sample Composition
Number Number of PSUs in the Population Number of 1/ of and Selected in the Sample Selected HHs (Sampling Munici- Population Sample Sample Mar-01* Dec-05 Fraction)* palities* Mar-01* Mar-01* Dec-05
Recife 14 3,068 261 283 4,715 5,610 200 Salvador 10 4,604 243 272 4,684 5,549 200 Belo Horizonte 33 14,710 359 389 6,644 7,544 200 Rio de Janeiro 19 20,612 406 441 7,576 8,309 500 S˜ao Paulo 39 3,023 431 471 7,820 9,119 700 Porto Alegre 30 4,982 329 378 5,773 6,763 200 Total 145 50,999 2,029 2,234 37,212 42,894
Notes: * Taken from IBGE (2002).
Another important feature of the PME sampling design is its rotation scheme. By design, each selected household is interviewed in four consecutive months. They are left out of the sample for eight consecutive months and return after this period to be interviewed again in four consecutive months, after which they are excluded from the sample. This characterises a rotating panel design known as 4-8-4. Ac- cording to this rotation scheme, 25% of the sample is substituted every month. In addition, surveys one year apart have 50% of households in common. This rotating design allows following selected households over time for a period of 16 months, with an 8 month gap between the fourth and fifth interviews.
Table 3.2 shows the representation of the rotation scheme for the year 2004. The cells of this table show the interview time for each panel and month. The super columns in Table 3.2 represent the selection groups3. In 2004 these selection groups were called D, E, F and G. Selection group D is not shown in the table. Each selection group is divided into eight sub-samples with approximately the same size, called rotation groups (RG). These are the eight columns (labelled from 1 to 8) in Table 3.2. The RGs are composed by a set of PSUs from any of the six metropolitan regions. The panels of households, which are rotated in and out, are represented by the combination of selection groups and RG. For example,
the panel of households F4 was first introduced in the survey in January 2004. In addition, once allocated to a RG, a PSU can only belong to this RG. In this sense, panels G4 and F4, for example, are composed by the same set of PSUs. However, the households allocated in selection group G are not the same as those allocated in F. In other words, each selected PSU has households allocated to different panels over the time that PSU is considered in the PME.
Table 3.2: The PME Rotation Scheme 4-8-4
Selection E Selection F Selection G 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 Jan 4 3 2 1 Feb 5 4 3 2 1 Mar 6 5 4 3 2 1 Apr 7 6 5 4 3 2 1 May 8 7 6 5 4 3 2 1 Jun 8 7 6 5 4 3 2 1 Jul 8 7 6 5 4 3 2 1 Aug 8 7 6 5 4 3 2 1 Sep 8 7 6 5 4 3 2 1 Oct 8 7 6 5 4 3 2 1 Nov 8 7 6 5 4 3 2 1 Dec 8 7 6 5 4 3 2 1
According to this rotation scheme, samples in consecutive months of the same year have 6 panels in common, and samples one year apart have 4 panels in common. This means that for every 12 months of data there is 50% of the sample that overlaps. Moreover, in every month there are households being interviewed from the first until the eighth time. This rotation scheme is the same as the one adopted by the CPS (Bureau of Labour Statistics, 2002). The use of rotating panel designs, like the one for the PME, over permanent samples is justified as it reduces the respondent’s burden. It also allows for longitudinal comparisons when a substantial number of household units are matched in consecutive months and years (Sedlacek et al., 1989).
The PME sample is designed such that households once excluded from the sample after the eighth interview, do not return to the survey in the following years. However, the PME, like most surveys of this type, is not a longitudinal survey per se. It is designed to follow household spaces (dwellings) but not individuals over time. If residents from a selected household move out from it, while the household is in the PME sample, no effort is made to follow these residents. The new occupants of this household will be those to be interviewed for the rest of the period that this household remains in the sample. This represents a drawback of this data set as one cannot guarantee individual records to be accurately linked across the distinct monthly surveys.
One other important aspect of the PME sampling design is that, unlike similar labour force surveys, in the Brazilian one, there is no change in the ques- tionnaire for the different months that the households are surveyed. Therefore, the selected household will respond to the exact same questionnaire in all eight times it is interviewed.