Nursing researchers are advised to consider the reliability and validity of psychologic instruments used in research; however, evaluation of physiologic, or clinical, instruments is not always given the same attention. Reliability and validity issues related to physiologic measures will be discussed. Terms commonly used for the evaluation of physiologic instruments, such as "accuracy," "precision," and "sensitivity," will be explained, with parallels drawn to traditional measurement terms. Sources of potential measurement error, such as environment, user, subject, machine, and interpretation error, will be explained and specific examples given from cardiopulmonary nursing research.