Cross-sectional studies are related to sample surveys. In both types of studies a sample S is taken from the target population U, for example by one of the sampling plans presented in Sect. 12.2. The basic difference between the two studies resides in their purpose and, consequently, in the way they are being evaluated. As remarked in Sect. 12.4, the purpose of a typical sample survey is to estimate indicators of variables separately for each variable of interest; in most surveys, these indicators have the form of a mean. For example in a household survey, we might want to estimate the average number of people in a household, the average income of a household, their average outlay for health care, the average number of consultations per year at their Commune Health Station etc.