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The quality of national demographic data in Africa is based on four factors: the completeness of coverage, the relevance of the data collection categories used, the reliability of the data collected, and how in-depth the data are on a given topic. This article focuses more specifically on the first two factors to examine why certain populations are under-enumerated in different types of national data collection. It draws on publicly available data: census data, demographic and health surveys (DHSs) and other national surveys and their documentation, including methodological reports and enumerator manuals. It identifies three groups that are under-enumerated or poorly represented—elderly women in the Sahel, nomadic pastoralists, and young men—and it examines the reasons for this: voluntary exclusion, data collection methodologies that are poorly adapted to the local culture, and harmonized definitions.