This application note, Part 3 of the Fundamentals of RF and Microwave Power Measurements, discusses the all-important theory and practice of expressing measurement uncertainty, mismatch considerations, signal flowgraphs, ISO 17025, and examples of typical calculations. Considerable detail is shown on the ISO 17025, Guide for the Expression of Measurement Uncertainties, has become the international standard for determining operating specifications. Agilent has transitioned from ANSI/NCSL Z540-1-1994 to ISO 17025.