Interpretation of Performance Scores

In Section 7.3.3, we learned about one of the most common scoring systems used in proficiency testing - the z-score - and how laboratory performance can be judged as being satisfactory, questionable or unsatisfactory. How should a laboratory act on the performance scores it receives Action should be considered in the following situations  

It is always useful to consider the performance in a proficiency testing round in a wider context. One of the main factors to consider is the performance of all of the participants in the round. If the majority of the results are satisfactory, but yours is not, this is likely to indicate a problem in your laboratory. However, it is worth remembering that your laboratory may have got the correct result and the other participants are in error In addition, if many other participants also have unsatisfactory results, there is still a problem, but it is less likely to be in your laboratory. 

Although the providers of proficiency testing schemes should have a quality management system in place, on occasions problems can arise which will affect the quality of the data evaluation being carried out. These can include transcription errors during data entry, mistakes in the report, software problems and inappropriate criteria for evaluation being used. Such problems should be remedied by the provider once the problem has been identified. 

Ultimately, it is up to the laboratory to carry out a thorough investigation when unsatisfactory scores are received. If all of the possibilities discussed above have been considered and ruled out as major contributions to the poor performance, the laboratory needs to look at its own procedures in some detail. There are many possible causes of unsatisfactory performance. They can generally be subdivided into two categories - analytical errors and non-analytical errors. Both are equally important. 

What do you think are common causes of analytical and non-analytical errors  

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