As discussed in Chapter 20, SOPs should be drawn up for QC at all stages of a trial, and, since the mapping and census are usually the first major field data collection stages of a trial, it is usually these steps that require the most preliminary pre-testing and pilot testing of all procedures, including post-enumeration checks and QC. After the census, the list of the population can be checked against other sources of information on the population. For example, school attendance records can be compared to the eligible age bands in the census, and the information collected can also be compared with other census data from the Central Statistics Office or elsewhere. Population pyramids can be drawn to see if there are any unusual features such as age heaping or disproportion- ate numbers of individuals at certain ages. Sex ratios can be checked, though one must allow for selective migration of certain age groups and of males or females.