Is the population coverage similar across the range of countries both in definition and execution? For many surveys the population is defined as residents in private households rather than the entire population so residents in communal establishments such as University Halls of Residence, residential care homes, prisons and hospitals are defined as out of scope. Is this important for your research question? Have any subgroups such as speakers of minority languages been left out from the survey as a whole or excluded from particular questions? Does the within household sample selection favour the head of household? Are geographically remote areas excluded or are hard to reach groups or difficult to interview groups excluded? Many surveys set a maximum non coverage rate - have these been met?

Even if the population appears to be similarly defined, for example, if the survey is of adults of working age the population definition will vary from country to country reflecting the legislation on retirement age. Similarly, for surveys of school children, the population in scope will be defined slightly differently taking account of differences in age of compulsory schooling. It is important to pay attention to issues of population definition and scope to ensure that you are making valid comparisons. This is an area that can easily be overlooked so familiarising yourself with the detail of the survey conditions is very important.

Within the survey there may be further narrowing of scope for particular questions and this may differ with some respondents routed past particular questions or sets of question. For example, in some countries, questions to women on sexual activity, fertility and family planning are only asked of women who are married.