Although writing clear questions is accepted as a general goal in surveys, procedures to ensure that each key term is consistently understood are not routine. Researchers who do not adequately test respondent understanding of questions must assume that ambiguity will not have a large or systematic effect on their results. Seven questions that were drawn from questions used in national health surveys were subjected to special pretest procedures and found to contain one or more poorly defined terms. When the questions were revised to clarify the definition of key terms, significantly different estimates resulted. The implication is that unclear terms are likely to produce biased estimates. The results indicate that evaluation of survey questions to identify key terms that are not consistently understood and defining unclear terms are ways to reduce systematic error in survey measurement.