The authors argue that while usability testers have drawn on demonstrated practices from a variety of the social and clinical sciences in developing their methods, they have not concerned themselves with the reliability and validity of the data produced. The authors suggest that the concepts of reliability and validity are relevant to usability testing and that a concern for reliability and validity will enhance the credibility and effectiveness of usability testers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>