At one extreme a case-control investigation may include fewer than 50 subjects, while at the other, some large longitudinal studies follow up many thousands of people for several decades. The main study designs will be described in later chapters, but we here discuss important features that are common to the planning and execution of surveys, whatever their specific design. Early planning The success of data collection requires careful preparation. The general interest has first to be translated into precisely formulated, written objectives. Every survey should be reasonably sure to give an adequate answer to at least one specific question.
Every study needs a primary purpose.
Background reading Before planning the detail of a study, it is wise to carry out a library search of the relevant background publications. Even when survey work remains necessary, experience in earlier related investigations may guide the design or indicate pitfalls to be avoided. The overriding need in an epidemiological survey is to examine a representative sample of adequate size in a standardised and sufficiently valid way. This determines the choice of examination methods and the points where these differ from those of clinical practice. Methods must be acceptable, and if possible noninvasive, or else cooperation suffers and the study group becomes unrepresentative.