South African HIV/AIDS, Behavioural Risks, Sero-status, and Mass Media Impact Survey (SABSSM) 2002: Visiting point data - All provinces

This project used the HSRC's master sample (HSRC 2002). A master sample is defined as a selection, for the purpose of repeated community or household surveys, of a probability sample of census enumeration areas throughout South Africa that are representative of the country's provincial, settlement and racial diversity. The sampling frame that was used in the design of the master sample was the 2001 census Enumerator Areas (EAs) from Statistics South Africa (Stats SA). The target population for this study were all people in South Africa, excluding persons in so-called special institutions (e.g. hospitals, military camps, old age homes, schools and university hostels). The EAs were used as the Primary Sampling Units (PSUs). Although the 2001 census data are not yet available, it was decided to use the 2001 EAs for the master sample because the sampling units would remain relevant for future surveys conducted by the HSRC within five to ten years' time. In addition, the HSRC would soon have access to the most recent census statistics over this period for weighting of future survey results, including this study. The sample was designed with two main explicit strata, namely, provinces and the geography type (geotype) of the EA. In the 2001 census, the four geotypes are urban formal, urban informal, rural formal (including commercial farms) and tribal areas (i.e. the deep rural areas). In the formal urban areas, race was also used as a third stratification variable. What this means is that the Master Sample has been designed to allow reporting of results (i.e. reporting domain) at a provincial, geotype and race level. A reporting domain is defined as that domain at which estimates of a population characteristic or variable should be of an acceptable precision for the presentation of survey results. The census 2001 EA data provided by Stats SA for drawing the sample contained an estimate of the number of dwelling units (DUs) or visiting points (VPs). A visiting point is defined as a separate (non-vacant) residential stand, address, structure, and flat in a block of flats or homestead. The 2001 estimate of visiting points was used as the Measure of Size (MOS) in the drawing of the sample. The visiting point is the Secondary Sampling Unit (SSU) in each of the selected PSUs. In this study, all people in all the households resident at the visiting point were initially listed, after which the eligible individual was randomly selected in each of the following three age groups 2-14, 15-24 and 25 years and older. These individuals constituted the Ultimate Sampling Units (USUs) of this study. Having completed the sample design, the sample was drawn with 1 000 PSUs or EAs being selected throughout South Africa (see Figure 2). These PSUs were allocated to each of the explicit strata. With a view to obtaining an approximately self-weighting sample of visiting points (i.e. SSUs), (a) the EAs were drawn with probability proportional to the size of the EA using the 2001 estimate of the number of visiting points in the EA database as a measure of size (MOS) and (b) to draw an equal number of visiting points (i.e. SSUs) from each drawn EA. An acceptable precision of estimates per reporting domain requires that a sample of sufficient size be drawn from each of the reporting domains. Consequently, a cluster of 11 VP was systematically selected on the aerial photography produced for each of the EAs in the master sample. Since it is not possible to determine on an aerial photograph whether a 'dwelling unit' is indeed a residential structure or whether it was occupied (i.e. people sleeping there), it was decided to form clusters of 11 dwelling units per PSU, allowing on average for one invalid dwelling unit in the cluster of 11 dwelling units. Previous experience at Statistics SA indicated a sample size of 10 households per PSU to be very efficient, balancing cost and efficiency. Overall, a total of 14 450 potential participants composed of 4 001 children, 3 720 youths and 6 729 adults were selected for the survey and 13 518 (93.6%) were actually visited. A small proportion (6.4%) of potential respondents could not be approached due to logistic constraints that were unavoidable in a study of such magnitude. Among the 13 518 individuals who were selected and contacted for the survey, 9 963 (73.7%) persons agreed to be interviewed, and 8 840 (65.4%) agreed to also give a specimen for an HIV test. The sample was designed with the view to enable reporting of the results on province level, on geography type area and on race of the respondent. The total sample size was limited by financial constraints, but based on other HSRC experience in sample surveys it was decided to aim at obtaining a minimum of 1 200 households per race group. In fact, the aim was to obtain 1 200 Indian households, 1 800 coloured households, 2 200 white households and 4 800 African households, a total thus of 10 000 households. The number of respondents per household for the study was expected to vary between one and three (one respondent in each of the three age groups). A 70% response rate was assumed and a HIV+ prevalence rate of 20%. However, the total refusal and noncontact rate was much higher than expected. Nevertheless, all cases where the interview could have been done were included in the analysis.

本项目采用南非人文科学研究理事会（Human Sciences Research Council, HSRC）2002年的主样本（master sample）。主样本指为开展重复社区或家庭调查，从南非全国范围内抽取的普查区（enumeration areas, EAs）概率样本，可代表该国各省、定居点类型与种族构成的多样性。本次主样本设计所使用的抽样框，源自南非统计局（Statistics South Africa, Stats SA）2001年普查的普查区数据。本研究的目标总体为南非境内所有民众，但排除所谓特殊机构中的人员，例如医院、军营、养老院、学校及大学宿舍等。普查区被用作初级抽样单元（Primary Sampling Units, PSUs）。尽管2001年普查的完整数据尚未公开，但研究团队仍决定采用该年度的普查区作为主样本的抽样单元，原因在于该抽样单元在未来5至10年内，均可适用于HSRC开展的各类后续调查。此外，在该时段内，研究团队将可获取最新的普查统计数据，用于包括本研究在内的后续调查结果的加权处理。本次样本设计设置两项核心显性分层变量，即省份与普查区的地理类型（geotype）。2001年普查将地理类型划分为四类：正规城市居住区、非正规城市居住区、正规农村居住区（含商业农场）及部落辖区（即深度农村区域）。在正规城市居住区中，种族被用作第三层分层变量。这意味着本主样本的设计可支持在省份、地理类型及种族层面进行结果报告（即报告域）。报告域指能够使人口特征或变量的估计值达到可接受精度，以供调查结果展示的统计域。南非统计局提供的2001年普查区抽样数据中，包含了住宅单元（dwelling units, DUs）或到访点（visiting points, VPs）的估算值。到访点的定义为独立的（非空置）住宅地块、地址、建筑及公寓楼内的公寓或住宅院落。研究采用2001年到访点的估算值作为样本抽取时的规模测度（Measure of Size, MOS）。每个被选中的初级抽样单元内，到访点均被用作次级抽样单元（Secondary Sampling Units, SSUs）。在本研究中，研究人员首先对所有位于到访点内的常住居民进行清单登记，随后从2-14岁、15-24岁及25岁及以上三个年龄组中随机抽取符合条件的个体，这些个体构成本研究的终极抽样单元（Ultimate Sampling Units, USUs）。完成样本设计后，研究团队在南非全国范围内抽取了1000个初级抽样单元（即普查区）（详见图2），并将这些单元分配至各显性分层中。为获取近似自加权的到访点（次级抽样单元）样本，研究采取两项措施：(a) 以2001年普查区数据库中的到访点数量估算值作为规模测度，按与普查区规模成正比的概率抽取普查区；(b) 从每个被抽取的普查区内抽取等量的到访点。为确保各报告域的估计值达到可接受精度，需从每个报告域中抽取足够规模的样本。因此，研究团队针对主样本中的每个普查区，在其航拍影像上系统抽取11个住宅单元组成的集群。由于无法通过航拍影像确认某一“住宅单元”是否为实际居住建筑，或是否有人员在此居住，研究团队决定在每个初级抽样单元内设置包含11个住宅单元的集群，平均预留1个无效住宅单元的余量。南非统计局过往经验表明，每个初级抽样单元抽取10户家庭的样本量能够在成本与效率间实现最优平衡。总体而言，本次调查共选取14450名潜在参与者，其中包括4001名儿童、3720名青年及6729名成年人，实际完成走访的人数为13518人，应答率为93.6%。受研究规模限制，存在无法避免的后勤约束，仅有6.4%的潜在受访者未能完成接洽。在13518名被选中并接洽的受访者中，9963人（73.7%）同意接受访谈，其中8840人（65.4%）同意提供人类免疫缺陷病毒（HIV）检测样本。本次样本的设计初衷为支持按省份、地理区域类型及受访者种族进行结果报告。总样本量受经费限制，但参考HSRC过往抽样调查的经验，研究团队设定的目标为每个种族群体至少获取1200户家庭样本。具体目标为：印度裔家庭1200户、混血裔家庭1800户、白人家庭2200户及非洲裔家庭4800户，总计目标样本量为10000户家庭。本研究预期每户家庭的受访人数为1至3人（对应三个年龄组各1名受访者）。研究预先假设70%的应答率及20%的HIV阳性率，但实际拒访与未接洽率远高于预期。尽管如此，所有可完成访谈的案例均被纳入分析。