All measurements have an uncertainty associated with it, meaning that the measurement result cannot be regarded as absolutely true or precise. This imprecision is defined as the uncertainty of measurement and is one of the most important quality characteristics of any measurement result. After adding a realistic uncertainty estimate to a measurement result, it makes possible the comparison between values to determine if in fact a particular measurement is different from another, or in the case of reference materials, differs significantly from a reference value. “Uncertainty of measurement does not imply doubt about the validity of measurement; on the contrary knowledge of the uncertainty implies increased confidence in the validity of the measurement result” EURACHEM / CITAC Guide CG4. The true value of a measurement will always remain unknown to the scientist. In other words, even though the scientist will always aim for the true value of a measurement, it is generally impossible to achieve it. This is due to the fact that real measurements are never made under perfect conditions. Errors and uncertainties can be derived from all types of sources: - The measuring instrument; including bias, ageing, wear, poor readability, etc. - The item being measured. - The measurement process. - Uncertainties such as calibration of the instrument and purity/accuracy of reagents. - Operator Skills. - Sampling Issues. - The environment; such as temperature, air pressure, humidity, etc. Where the size and effect of an error are known a correction can be applied to the measurement result. In general, uncertainties from each of these sources, and from other sources, would be individual “inputs” contributing to the overall uncertainty in the measurement.