Laboratory analytes for individuals are subject to several sources of variation, including biological, preanalytical (specimen collection), analytical (bias and imprecision), and postanalytical variation (reporting of results). Laboratory analytical bias is the closeness of an analyte result to the ‘‘true value’’ of the result, and the true value of a laboratory result is established by a ‘‘gold standard’’ method using standard reference materials. Bias is a measure of the systematic error of laboratory measurements. Precision is the repeatability of an analyte result if the same sample is tested many times, and the imprecision of an analyte is a measure of random errors. Biological variation consists of within-person (WP) and between-person (BP) variations, which are used to set analytical goals for bias and imprecision, assess serial changes in individual analytes, and gauge the clinical utility of population-based reference intervals.