Key Concepts About Non-probability Based Environmental Chemical Data Files
Some environmental analytes from NHANES were measured in a non-probability-based or "convenience” sample. Inference from these analyses is limited to those respondents in the sample and they are not representative of the U.S. population. During NHANES III, a convenience sample was used because the decision to include measurements for some environmental chemicals was made after original NHANES protocol was established. Surplus specimens from the NHANES III convenience sample have been used in the development of new laboratory protocols.
A convenience sample differs from the probability-based sample used for NHANES. A convenience sample consists of persons whose probability of selection is unknown. In other words, it is not possible to know how representative the sample is of the population from which it is selected. In contrast, persons selected into probability samples have a known probability of their selection, and inferences can be drawn from it to the population from which it is drawn (taking into account the probabilities of selection). In other words, the sample is representative of the population from which it is drawn.