Ensuring the Validity of Results

Measurement results are the foundation of decision-making; thus, their validity is fundamental. Ensuring the validity of results is a mandatory requirement of quality systems for testing and calibration laboratories. Laboratories ensure validity by external and internal measures. Although external measures give impartial and objective evidence on the validity of results, internal measures ensure validity daily. All measures for ensuring the validity of results monitor two performance parameters: precision as within-laboratory reproducibility and trueness as bias. Bias is checked and controlled by certified reference materials, while precision is checked and controlled by repeated measurements of reference materials, quality control materials, and routine test samples. The use of certified reference materials is essential as it establishes the metrological traceability. Control charts are used for long-term monitoring of bias and precision and to prove that method performances are stable and under statistical control. Consistency of results can be determined by using alternative instrumentations, replicate testing, sample retesting, different methods, correlations between different sample characteristics, and replicate evaluation of results. Instruments are checked by working standards and during functional and intermediate checks. Personnel can also be monitored through intralaboratory comparisons and proficiency testing. All these data can also be used to assess and monitor measurement uncertainty. This chapter describes all these tools, methods, and basic guidelines for ensuring the validity of results while presenting real-life laboratory examples.