Botswana - Botswana AIDS Impact Survey III 2008

Sampling Frame
For BAIS-II the sampling frame is based on the 2001 Population and Housing Census. This comprised the list of all Enumeration Areas (EAs) together with number of households.
Stratification
All districts and major urban centres become their own strata. EAs were grouped according to ecological zones in rural districts and according to income categories in cities/towns. Geographical stratification along ecological zones and income categories was undertaken to improve the accuracy of the survey data because of the homogeneity of the variables within each stratum
Sample Design
A stratified two-stage probability sample design was used for the selection of the sample.The first stage was the selection of EAs as Primary Sampling Units (PSUs) selected with probability proportional to measures of size (PPS), where measures of size (MOS) were the number of households in the EA as defined by the 2001 Population and Housing Census. In all 459 EAs were selected with probability proportional to size.In the second stage of sampling, the households were systematically selected from a fresh list of occupied households prepared at the beginning of the survey's fieldwork (i.e. listing of households for the selected EAs). Overall 8,275 households were drawn systematically.


Once the data set was cleaned, sampling weights were applied to the data. Being a multistage design, it follows naturally that the sample selected at each stage represents (or is assumed to represent) the respective population. The fundamental assumption is that units selected at each stage were similar to those not selected, in respect of characteristics of interest. In the treatment of units for the non-response the assumption that the responders were similar to non-respondents though should not be always taken for granted. Sampling weights are equal to the inverse of the probability of selection. Therefore the sampling probabilities at first stage of selection of EAs including probabilities of selecting the households were used to calculate the design weights. Non response adjustments were also taken into consideration when calculating the final sampling weight.