After the target population has been defined, the sampling strategy must be chosen. The first step is to define the sampling unit . Is it a subject?, a family unit? or a home?, or a city block? or even a whole village?. The sampling strategy depends of the type of population listing we have, the so called Sampling frame. Do we have an electoral list? or a list of students in a school? Or we must produce our own listing of house addresses?
Do we want a cross-sectional or a follow-up study?