This guideline covers the design of measurement procedure comparison experiments using patient samples and subsequent data analysis techniques used to determine the bias between two in vitro diagnostic measurement procedures. Clinical and Laboratory Standards Institute guideline EP09 - Measurement Procedure Comparison and Bias Estimation Using Patient Samples is written for laboratorians and manufacturers. It describes procedures for determining the bias between two measurement procedures, and it identifies factors for consideration when designing and analyzing a measurement procedure comparison experiment using patient samples. An overview of the measurement procedure comparison experiment includes considerations for both manufacturers and laboratorians. Details on how to create difference and scatter plots for visual inspection of the data are provided. Once the data are characterized, various methods are introduced for quantifying the relationship between two measurement procedures, including bias estimates and regression techniques. The final chapter contains recommendations for manufacturers' evaluation of bias and statement format for bias claims.