4.1. Errors

Any survey will be vulnerable to errors which may invalidate the extrapolation of the sample results to the target population. The most important sources of error are discussed in detail below. The aim in each investigation must be to minimise the non-sampling errors, and to quantify as far as possible the (unavoidable) sampling error. Therefore we describe these various sources of error in detail, as they should be borne in mind at the planning stage of the survey.