Clinical and Laboratory Standards Institute document EP27-P--How to Construct and Interpret an Error Grid for Diagnostic Assays; Proposed Guideline describes what an error grid is, why it is useful, and how to go about constructing it. Error grids inform users about the performance required to prevent potential patient harm. Once constructed, error grids can be populated with data from a method comparison experiment. Then one can calculate the proportion of data in each error grid zone as well as confidence intervals. Three examples are provided.