An estimate of variation is needed to determine sample size. Pretesting gives an estimate of a population's willingness to participate in research. Pretesting also allows researchers to determine if there is enough variation in responses to analyze them meaningfully. Are response categories inclusive of all answers in the population. Avoid depleting or polluting main sample.
Pollution can be diffusion of instrument content.
If you have a close or closed population this can take the form of discussion among potential subjects and the pretest group. Depletion can be using up a pool of legitimate samples. Ensure sufficient heterogeneity in pretest sample to allow for the broadest range of responses. They should represent the full range of variation . Sampling Strategies Sample Size, Probability Samples Ideally, using a census of the population, use a selection technique that guarantees that every member of the population has an equal chance of being included in the sample.