South African National HIV Prevalence, HIV Incidence, Behaviour and Communication Survey (SABSSM) 2008: Guardian 2-11 - All provinces

As in previous surveys, a multi-stage disproportionate, stratified sampling approach was used. A total of 1 000 census enumeration areas (EAs) from the 2001 population census were selected from a database of 86 000 EAs and mapped in 2007 using aerial photography to create a new updated Master Sample as a basis for sampling visiting points/households. The selection of EAs was stratified by province and locality type. Locality types were identified as urban formal, urban informal, rural formal (including commercial farms), and rural informal. In the formal urban areas, race was also used as a third stratification variable (based on the predominant race group in the selected EA at the time of the 2001 census). The allocation of EAs to different stratification categories was disproportionate; that means, over-sampling or over-allocation of EAs was done, for example, in areas that were dominated by Indian, coloured or white race groups to ensure that the minimum required sample size in those smaller race groups was obtained. The Master Sample was 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. 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. A maximum of four visits were made to each VP to optimise response. Fieldworkers enumerated household members, using a random number generator to select the respondent and then preceded with the interview. All people in 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: under 2 years, 2-14 years, 15-24 years and 25+ years. These individuals constituted the USUs of this study. Having completed the sample design, the sample was drawn with 1 000 PSUs or EAs being selected throughout South Africa. 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 15 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 15 dwelling units per PSU, allowing on average for one invalid dwelling unit in the cluster of 15 dwelling units. Previous experience at Statistics SA indicated a sample size of 10 households per PSU to be very efficient, balancing cost and efficiency. The VP questionnaire was administered by the fieldworker, and in follow-up, participant selection was made by the supervisor. Participants aged 12 years and older who consented were all interviewed and also asked to provide dried blood spots (DBS) specimens for HIV testing. In case of 0-11 years, parents/guardians were interviewed but DBS specimens were obtained from the children. The sample size estimate for the 2008 survey was guided by the (1) requirement for measuring change over time in order to detect a change in HIV prevalence of 5 percentage points in each of the main reporting domains, namely gender, age-group, race, locality type, and province (5% level of significance, 80% power, two-sided test), and (2) the requirement of an acceptable precision of estimates per reporting domain; that is, to be able to estimate HIV prevalence in each of the main reporting domains with a precision level of less than 4%, which is equivalent to the expected width of the 95% confidence interval (z-score at the 95% level for two-sided test). A design effect of 2 was assumed. Overall, a total of 20826 interviewed participants composed of 4981 children (0-14 years), 5344 youths (15-24 years) and 10501 adults (25+ years) were interviewed. 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. 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). More females (68.9%) than males (62.02%) were tested for HIV. The 25+ years age group was the most compliant (68.8%), and 2-14 years the least (58.9%). The highest testing response rate was found in urban informal settlements (72.5%) and the lowest in urban formal areas (62.8%).

与既往调研一致，本研究采用多阶段非比例分层抽样方法。研究从包含86000个普查枚举区（Census Enumeration Areas, EAs）的数据库中，选取2001年人口普查的1000个普查枚举区，并于2007年通过航空摄影完成绘图，以此构建更新后的主样本（Master Sample），作为抽取访问点/住户的基础。普查枚举区的分层按省份和居住类型展开。居住类型被划分为正规城市地区、非正规城市地区、正规农村地区（含商业农场）以及非正规农村地区。在正规城市地区中，种族被作为第三层抽样变量（基于2001年普查时所选普查枚举区内的主导种族群体）。普查枚举区在不同分层类别中的分配采用非比例方式：即对印度裔、有色人种或白人主导的区域进行过度抽样/过度分配，以确保这些规模较小的种族群体能够达到所需的最小样本量。本主样本的设计旨在支持按省份、地理类型以及种族维度报告调研结果（即报告域）。报告域的定义为：在该域下，人口特征或变量的估计值需达到可接受的精度，以满足调研结果的呈现需求。访问点（Visiting Point, VP）指独立的（非空置）住宅地块、地址、建筑以及公寓楼或宅地中的公寓单元。研究以2001年估算的访问点数量作为规模测度（Measure of Size, MOS），用于样本抽取。为优化应答率，每个访问点最多进行4次回访。调研人员枚举住户家庭成员，通过随机数生成器选取受访对象后开展访谈。首先列出所有居住在该访问点的住户成员，随后按以下四个年龄组分别随机选取符合条件的个体：2岁以下、2-14岁、15-24岁以及25岁及以上。这些个体构成本研究的研究单元（USUs）。完成样本设计后，在南非全境抽取1000个初级抽样单元（Primary Sampling Units, PSUs）即普查枚举区，并将这些初级抽样单元分配至各明确分层。为获得近似自加权的访问点（即次级抽样单元Secondary Sampling Units, SSUs）样本，采取以下两步：(a) 以2001年数据库中各普查枚举区内的访问点数量估算值作为规模测度，按与普查枚举区规模成正比的概率抽取普查枚举区；(b) 从每个被抽取的普查枚举区内抽取等量的访问点（即次级抽样单元）。为使各报告域的估计值达到可接受的精度，需从每个报告域中抽取足够规模的样本。因此，针对主样本中的每个普查枚举区，在其配套航空摄影图上系统选取15个访问点作为群集。由于无法通过航空摄影判断某一‘居住单元’是否为住宅建筑，亦或是否有人居住（即是否有人员在此过夜），因此设定每个初级抽样单元对应的群集包含15个居住单元，平均每15个居住单元中允许存在1个无效居住单元。南非统计局（Statistics SA）的既往经验表明，每个初级抽样单元选取10户住户的样本量在成本与效率间达到了极佳平衡。调研人员向受访对象发放访问点问卷，后续的参与者选取由督导完成。年满12岁且同意参与的受访者均接受访谈，并被要求提供干血斑（Dried Blood Spots, DBS）样本用于HIV检测。对于0-11岁的儿童，由其父母/监护人接受访谈，但采集儿童的干血斑样本。2008年调研的样本量估算基于以下两项要求：(1) 实现跨时间变化的测量，以在各主要报告域（即性别、年龄组、种族、居住类型以及省份）中检测出5个百分点的HIV感染率变化（显著性水平5%，检验功效80%，双侧检验）；(2) 各报告域的估计值需达到可接受的精度：即各主要报告域的HIV感染率估计精度需小于4%，这等价于95%置信区间（双侧检验95%水平z值）的预期宽度。研究假设设计效应（Design Effect）为2。最终总计完成20826名参与者的访谈，其中包括4981名儿童（0-14岁）、5344名青年（15-24岁）以及10501名成人（25岁及以上）。本样本的设计旨在支持按省份、地理类型区域以及受访者种族维度报告调研结果。总样本规模受经费约束，但基于南非人类科学研究委员会（Human Sciences Research Council, HSRC）在抽样调研中的既往经验，研究设定的目标为每个种族群体至少获取1200户住户。本研究中每户的受访对象数量预计为1至3名（即三个年龄组各1名受访者）。HIV检测的女性参与者占比（68.9%）高于男性（62.02%）。25岁及以上年龄组的依从性最高（68.8%），2-14岁年龄组的依从性最低（58.9%）。检测应答率最高的群体为非正规城市定居点（72.5%），最低的为正规城市地区（62.8%）。