Sampling Procedure
The sample is based on the 1996 Population Census which recorded a total population of 728 916 people in the Mitchell's Plain Magisterial district. The survey sampled households in the designated geographic area using a two-stage cluster sample. The first stage of this sample entails selecting clusters of households and the second stage entails the selection of the households themselves. For the clusters of households, the survey relied on the Enumerator Areas as defined by Statistics South Africa for the 1996 Population Census. Enumerator Areas are neighbourhoods of roughly 50 to 200 households and drawn up by the Chief Directorate of Demography at Statistics South Africa. This directorate is responsible for developing and maintaining a GIS system that provides the maps that are used for conducting the five-yearly national population census (Statistics South Africa, 2001:42-44). Although Enumerator Area boundaries do not cross municipal boundaries, they do not correspond to any other administrative demarcations such as voting wards. Enumerator Areas are designed to be homogeneous with respect to housing type and size. For example, Enumerator Area boundaries within the Mitchell's Plain Magisterial District do not usually cut across different types of settlements such as squatter camps, site and service settlements, hostels, formal council estates or privately built estates. Instead, each Enumerator Area is homogeneous with respect to any one of these housing types.
The method of selection used was that of Probability Proportional to Size (PPS), where size is measured as the number of households in each Enumerator Area (as defined by the 1996 Population Census). This method provides the most efficient way to obtain equal sub-sample sizes across two stages of selection, i.e. to select the Enumerator Areas and then select from each Enumerator Area a constant number of households for all Enumerator Areas in the sample. The sample is implicitly stratified by location and by housing type.
A more detailed description of the sampling method and procedure for this survey can be found in the sampling method document available in the zipped folder under 'documentation'.
Weighting
Post Stratification and weighting for non-response:
Non-response can lead to an increase in sample errors and bias in estimates. The weight pweight2, included in the data, is designed to correct for households who were selected into the sample, but were not interviewed. This could have been due to refusals, failure to locate the household or not being able to make contact with the residents.
The weight pweight2, included in the data, adjusts for this type of nonresponse. Pweight2 was constructed by adjusting the original weight, pweight1 by the inverse of the response rate in each Enumeration area. For example, if the response rate is 80 percent then a suitable weight would be 1/0.80 =1.25. This was done for each numerator Area and applied to each responding household. (Under such a method of dealing with non-response it is assumed that all households selected into the sample have the same probability of responding.)
Post Stratification
Non-response in the survey also occurs at the individual level i.e. there are cases where not all adults within a household were interviewed. Nonresponse at the individual level is corrected for by post stratifying the data according to known age, gender and race proportions as reflected in the the 1996 population census. The raking ratio method of post stratifying weight adjustment was used to calculate and make adjustments to the pweight2 variable in the data. This results in the variable, adultrakingweight, in the data. When analysing data in the adult file of the data the post stratified weight, adultrakingweight, should be used to adjust for adult non-response in the survey.
All three weights are available to the analyst:
• pweight1:-the original weight with no adjustments for nonreponse.
• pweight2:- pweight1, adjusted for household non-response.
• Adultrakingweight:-post stratified weights to adjust for adult nonresponse.
It is suggested that the post stratified weights be applied when doing most analysis.
Attention is drawn to the fact that the survey was completed in the year 2000, whereas the auxiliary information used to make the post stratification adjustments come from the 1996 population census. It is likely that some change in population proportions may have occurred between 1996 and 2001. It is our intention to use the 2001 census data to further adjust the weights, when this information becomes available.