In order to end up with a representative sample of sufficient size to perform the study, we need to define several elements:

### What is the sampling unit?

This is the smallest element of the sampling population in which we will make the observations of the study. Usually this is an individual person; the obeservations we make by questionnaires or interviews targeted at individual members of the study population. However it could be a household, a school, or even entire towns or cities.

For example if we want to study the relationship between religion and vaccine coverage, this would be done by studying "the population of towns in the country" and to measure within each town the distribution (proportions) of various religions and the vaccine coverage.

### What is the sampling fraction?

This is the proportion of the target population that is included in the sample. For example, if the total size of the target population that we want to study is 100.000 people, and in the sample we include 500, then the sampling fraction is 0.5%