R&R studies to help multi-location companies!

Inter-laboratory assessment

A laboratory functioning either on quality control of a processing plant, or as a stand-alone unit, needs to be assessed for the efficiency of the results it produces. This same necessity exists when a new test method should be examined against the reference method. The tests usually employed at this end are part of a "ring test" with other qualified laboratories, and the basic performance measure for every analysis method is the repeatability and the reproducibility.

The term repeatability describes the closeness of the agreement between the results of successive measurements of the same sample carried out under the same conditions of measurement. Repeatability conditions include: the same measurement procedure, the same observer, the same measuring instrument, used under the same conditions, the same location, repetition over a short period of time. Repeatability may be expressed quantitatively in terms of the dispersion characteristics of the results.

Reproducibility expresses closeness of the agreement between the results of measurements of the same sample carried out under changed conditions of measurement. A valid statement of reproducibility requires specification of the conditions changes. The latter may include: principle of measurement, method of measurement, observer, measuring instrument, reference standard, location, conditions of use, time. Reproducibility may be expressed quantitatively in terms of the dispersion characteristics of the results.

The new analysis method (or instrument, laboratory, analyst, etc.) is 'valid' if the results obtained by the different laboratories fall within the expected uncertainty limit. Deviations beyond such limits may indicate for example a lack of control of the influencing parameters. The causes of this behaviour should be clarified and the method is to be redefined accordingly.

Inter-laboratory testing schemes involve randomly selected sub-samples from a source of material being distributed simultaneously to participating testing laboratories for concurrent testing. After completion of the testing, the results are returned to PES, and compared with the assigned value(s) to give an indication of the performance of the individual laboratories and the group as a whole.

During the evaluation of performance, we are responsible for ensuring that the method of evaluation is appropriate to maintain the credibility of the scheme. The items that are analysed are: the overall performance versus prior expectations (taking uncertainties into account), the variation within and between laboratories (together with comparisons with any published precision data), and the possible sources of error with the suggestions for improving performance.

More information