To reduce the time required for the survey, respondents are assessed in only one or two of the three domains, not in all of them. Respondents with very low literacy skills take an alternative assessment of basic reading skills. The survey is given in the official language or languages of each participating country, sometimes also including a widely-spoken minority or regional language. Sample sizes depend on the number of cognitive domains assessed, the number of languages used, and decisions by countries about whether to increase the sample sizes to allow more precise estimates for individual geographic regions or population subgroups.