ABSTRACT
This chapter discusses undercoverage, sample selection, nonresponse, and measurement errors. Undercoverage occurs in a poll if the sampling frame only contains a part of the target population. It is important to realize that the undercoverage bias of an online poll does not depend on the sample size. Sample selection must be such that every person in the target population has a nonzero probability of selection, and all these probabilities must be known. The principles of probability sampling also apply to online polls. Nonresponse occurs when persons in the selected sample, who are also eligible for the poll, do not provide the requested information. The problem of nonresponse is that the availability of data is determined both by the sample-selection mechanism and the response mechanism. Undercoverage, self-selection, and nonresponse can all affect the representativity of the response of an online poll. Measurement error is defined as the difference between the recorded answer to a question and the correct answer.
