The response weights are calculated as part of the GEM APS harmonization process using information on the gender of the respondent and their age coded in normal GEM age groups. The age-gender distribution of the sample population is then compared to a known distribution for the country and weighting factor calculated so that the distribution in the sample matches that of the adult population for the country. These resulting weights are then scaled so that they have an average value of 1.0. This means that age-gender combinations that are under-represented in the sample as compared to the population will have a weight of great than 1.0, and those that are over-represented in the sample will have a weight of less than 1.0.
Response weights are derived as follows. The APS sample data is tabulated into a two-way table by gender and age group, with each cell representing the proportion of the sample represented by that age-gender combination (e.g., one cell would have the proportion of the sample that are female aged 45 to 54). The number of age groups in this table can vary from country to country, depending on the age range selected for the sample (e.g., ages 18 to 64 versus ages 18 to 99). The reference population is tabulated into a two-way table using the exact same cell definitions. A third table is then calculated, using the same two-way table structure of age by gender, where each cell is the ratio of the sample proportion to the reference population proportion for that age-gender combination.
Thus, for example, if females aged 45 to 54 represent 11.56% of the collected sample, and represent 11.78% of the reference population; this third table would contain the value 1.0187 = 11.78% / 11.56%. This value is then the RAW WEIGHT for females aged 45 to 54. This process is repeated for every cell in the table. Once all the raw weights have been computed, they are summed to determine their total of all the weights, which is then divided by the number of cases in the sample, to scale all the RESPONSE WEIGHTS to have an average weight of 1.0.
Two weights are provided in the database:
WEIGHT_L includes weights that match the distribution of the adult population between 18-64 years. Hence, individuals in the sample who are 65 or older have a missing value (SYSMIS, indicated by a ‘.’ in the SPSS file) and will be excluded from any analysis using WEIGHT_L.
WEIGHT_A is very similar but includes weights that match the distribution of the adult population 18+ years. If the sample design of a country is such that it includes only individuals between 18-64 years, WEIGHT_L will be identical to WEIGHT_A.
The data used for calculating GEM APS weights comes from either the official population data submitted by the National Team as part of its GEM APS proposal or from the Midyear Population, by Age and Sex from U.S. Census Bureau, International Data Base.