This document discusses uncertainty of measurement in testing and calibration. It explains that measurement values can vary between repeated measurements due to factors like human error, instruments, materials, test methods, calibration, and environment. Laboratories must estimate measurement uncertainty as required by ISO17025, taking into account test method requirements, customer requirements, and conformity limits. There are two types of uncertainty - Type A is evaluated statistically from repeated measurements, while Type B is evaluated from calibration certificates, specifications, and experience. Examples are given of calculating uncertainty for different measurement methods and instruments. Microbiological testing poses additional challenges for uncertainty estimation due to non-uniform sample distribution and qualitative results.