Afrobarometer uses national probability samples designed to meet the following criteria. Samples are designed to generate a sample that is a representative cross-section of all citizens of voting age in a given country. The goal is to give every adult citizen an equal and known chance of being selected for an interview. We achieve this by:
- using random selection methods at every stage of sampling;
- sampling at all stages with probability proportionate to population size wherever possible to ensure that larger (i.e., more populated) geographic units have a proportionally greater probability of being chosen into the sample.
The sampling universe normally includes all citizens age 18 and older. As a standard practice, we exclude people living in institutionalised settings, such as students in dormitories, patients in hospitals, and persons in prisons or nursing homes. Occasionally, we must also exclude people living in areas determined to be inaccessible due to conflict or insecurity. Any such exclusion is noted in the technical information report (TIR) that accompanies each data set.
Sample size and design
Samples usually include either 1,200 or 2,400 cases. A randomly selected sample of n=1200 cases allows inferences to national adult populations with a margin of sampling error of no more than +/-2.8% with a confidence level of 95 percent. With a sample size of n=2400, the margin of error decreases to +/-2.0% at 95 percent confidence level.
The sample design is a clustered, stratified, multi-stage, area probability sample. Specifically, we first stratify the sample according to the main sub-national unit of government (state, province, region, etc.) and by urban or rural location.
Area stratification reduces the likelihood that distinctive ethnic or language groups are left out of the sample. Afrobarometer occasionally purposely oversamples certain populations that are politically significant within a country to ensure that the size of the sub-sample is large enough to be analysed. Any oversamples is noted in the TIR.
Samples are drawn in either four or five stages:
Stage 1: In rural areas only, the first stage is to draw secondary sampling units (SSUs). SSUs are not used in urban areas, and in some countries they are not used in rural areas. See the TIR that accompanies each data set for specific details on the sample in any given country.
Stage 2: We randomly select primary sampling units (PSU).
Stage 3: We then randomly select sampling start points.
Stage 4: Interviewers then randomly select households.
Stage 5: Within the household, the interviewer randomly selects an individual respondent. Each interviewers alternates in each household between interviewing a man and interviewing a woman to ensure gender balance in the sample.
To keep the costs and logistics of fieldwork within manageable limits, eight interviews are clustered within each selected PSU.
For some national surveys, data are weighted to correct for over or under-sampling or for household size. “Withinwt” should be turned on for all national –level descriptive statistics in countries that contain this weighting variable. It is included as the last variable in the data set, with details described in the codebook. For merged data sets, “Combinwt” should be turned on for cross-national comparisons of descriptive statistics. Note: this weighting variable standardizes each national sample as if it were equal in size.
Further information on Round 6 sampling protocols, including full details of the methodologies used for each stage of sample selection, can be found in Section 5 of the Afrobarometer Round 6 Survey Manual.
In Rounds 1 through 3, the Afrobarometer did only minimal weighting of data to correct for over- or under-samples of certain populations, usually based on region or urban-rural location. Starting in Round 4, however, we began collecting additional data (population of each Enumeration Area [EA] selected and the total population of each stratum) in order to improve our calculations of weighting factors based on individual selection probabilities, which are now included for all countries. This allows us to compute much more comprehensive and accurate within-country weights, which can be identified by the variable “WITHINWT”.