In biomedical measurement, comparisons between two instruments or betw
een a new instrument and the gold standard is commonly needed to evalu
ate the reliability and validity of the instruments. Such studies are
frequently analyzed using statistics that are inappropriate for studyi
ng concordance. This article reviews the appropriate statistical techn
iques and provides a current methodological guide for the evaluation o
f reliability and validity of instruments used in the measurement of b
iomedical variables. The techniques are illustrated using real data fr
om a pilot study comparing four instruments for measuring cholesterol.