Reliability can have different meanings when applied to human performance, equipment and measurement (Crowder et al., 2000: 1). Reliability of equipment and systems is concerned with the probability that the system is working successfully. Typical measures used in the evaluation of this type of reliability are mean time between failure and rate of failure occurrence (Sommerville, 1992: 394–5). The reliability of a human performer could be the ability of the person to consistently perform specified tasks to a required standard. This chapter is concerned with the reliability of measurement within scientific research. The chapter commences by covering the measurement issues of validity, objectivity and reliability ...