A map of the trial area and a population enumeration (census) provide:
- a sampling frame for the selection of those in the target population who will be included in a trial
- denominators for the computation of morbidity and mortality rates
- baseline population characteristics, which may affect the impact of an intervention and which can also be monitored for changes during the study
- a basis for planning the logistics of the fieldwork, for example, which households should be visited by one fieldworker and in which order or to demarcate clusters within a cluster randomized trial
- a means for studying factors that affect disease rates. Age, sex, and place of residence affect the risk of many diseases, and information on these and other factors that may influence exposure or susceptibility to disease, or which may influence its outcome, should be recorded at the start of a trial.