Clinical and Laboratory Standards Institute document EP27-A - How to Construct and Interpret an Error Grid for Quantitative Diagnostic Assays; Approved Guideline describes what an error grid is, why it is useful, and how to construct it. An error grid illustrates the relationship between results obtained by one quantitative test to those obtained by a second one, while considering the diagnostic or therapeutic implications of the magnitude of the difference between the two results. Error grids inform users about the performance required to prevent potential patient harm. Once constructed, error grids can be populated with data from a measurement procedure comparison experiment. The proportion of data in each error grid zone is used to evaluate the clinical effectiveness of the measurement procedure.