Probability-Based Sampling for Human Exposure Assessment Studies
Whenever one studies a sample of people to determine the effects of environmental exposures on them, one usually wants to extrapolate the findings beyond the individuals actually studied and measured. This extrapolation from study subjects to the population represented by them requires application of inferential statistics. A firm foundation for inferential statistics is established by using the scientific method to design and implement the study. The scientific method requires one to explicitly define the population about which one wants to make inferences and to select a sample from that population in such a manner that the probability of being selected into the sample is known for every person selected into the sample. Sampling procedures that result in known probabilities of selection from a specified finite population are referred to as probability-based sampling methods. The purpose of this chapter is to give you a basic understanding of what probability-based sampling methods are, when they should be used, how they are applied, when it may be satisfactory to not use them, and what else is required to support defensible inferences from a sample to the population it represents. For example, it is important that the size of the
sample be large enough and that valid measurements be obtained for a high proportion of the people selected into the sample.