Proficiency testing scheme

What do you think is the main difference between a proficiency testing scheme and a collaborative study  

A proficiency testing scheme tests the performance of the participating laboratories whereas a collaborative study is used to test the performance of a particular method. 

These two different types of interlaboratory study are discussed in detail in the following sections. 

There are two main types of proficiency testing scheme. First, there are those set up to assess the competence of a group of laboratories to undertake a very specific analysis, e.g. lead in blood or the number of asbestos fibres in air collected on membrane filters. Secondly, there are those schemes used to evaluate the performance of laboratories across a certain sector for a particular type of analysis. Because of the wide range of possible analyte/matrix combinations it is not practicable to assess the performance of laboratories when analysing all the possible sample types. Instead, a representative cross-section of analyses is chosen (e.g. determination of different pesticide residues in a range of foodstuffs or the determination of trace levels of metals in water samples). 

Each of these two main types of proficiency testing schemes can be further subdivided into three categories, as follows  

The results obtained by the laboratories participating in a PT scheme are most commonly expressed as z-scores, where z is given by (see Section 2.2)  

The target value for the standard deviation, o, should be circulated in advance to the PT participants along with a summary of the method by which it has been established. It will vary with anal)4e concentration, and one approach to estimating it is to use a functional relationship between concentration and standard deviation. The best-known relationship is the Horwitz trumpet, dating from 1982, so called because of its shape. Using many results from collaborative trials, Horwitz showed that the relative standard deviation of a method varied with the concentration, c (e.g. mg g ), according to the approximate and empirical equation  

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The principle of proficiency testing schemes consists in analyzing one or more samples sent to the laboratories by an external body. The analytical results returned to the organizer are evaluated in comparison to the assigned value(s) of the sample(s). 

Take part in External Quality Assessment, formally through Proficiency Testing schemes and informally by taking part in intercomparison studies. 

The equation representing this curve was introduced in Section 4.4 (equation (4.4)). However, a more contemporary model based on results from Proficiency Testing schemes has shown that the relationship is best represented if three equations are used to cover from high to low concentrations, as shown in... 

This chapter deals with handling the data generated by analytical methods. The first section describes the key statistical parameters used to summarize and describe data sets. These parameters are important, as they are essential for many of the quality assurance activities described in this book. It is impossible to carry out effective method validation, evaluate measurement uncertainty, construct and interpret control charts or evaluate the data from proficiency testing schemes without some knowledge of basic statistics. This chapter also describes the use of control charts in monitoring the performance of measurements over a period of time. Finally, the concept of measurement uncertainty is introduced. The importance of evaluating uncertainty is explained and a systematic approach to evaluating uncertainty is described. 

To understand the difference between proficiency testing schemes and collaborative studies. 

To be familiar with the different approaches to obtaining the assigned value and target range for a proficiency testing scheme. 

Figure 7.1 Sample distribution schemes for proficiency testing schemes (a) single sample distribution (b) split-sample testing.

Figure 7.1(a). This is by far the most common type of proficiency testing scheme. 

There are numerous proficiency testing schemes available, operated by a number of different organizations, Table 7.1 gives some examples. Further information on the range of proficiency testing schemes available can be found on EPTIS, a web-based Proficiency Testing information system. This is a database of PT schemes covering most measurement sectors (www.eptis.bam.de). 

Table 7.1 Examples of proficiency testing schemes organized by UK providers...

Irrespective of the type of proficiency testing scheme, it is usually organized in a sequence of clear steps. 

There is no experimentally established optimum frequency for the distribution of samples. The minimum frequency is about four rounds per year. Tests that are less frequent than this are probably ineffective in reinforcing the need for maintaining quality standards or for following up marginally poor performance. A frequency of one round per month for any particular type of analysis is the maximum that is likely to be effective. Postal circulation of samples and results would usually impose a minimum of two weeks for a round to be completed and it is possible that over-frequent rounds have the effect of discouraging some laboratories from conducting their own routine quality control. The cost of proficiency testing schemes in terms of analysts time, cost of materials and interruptions to other work has also to be considered. 

The operation of proficiency testing schemes is described in the harmonized protocol produced by IUPAC [1]. Further information can be found in ISO/IEC Guide 43 [2] and ILAC Guide 13 [3],... 

All proficiency testing schemes should have a statistical protocol which states clearly how the data will be processed and how laboratory performance will be evaluated. This protocol should also describe how the assigned value for any parameter in a test sample is estimated. This is an important consideration, as the performance of individual laboratories is gauged by comparison with the assigned value. 

There are a number of different performance scoring systems used in proficiency testing schemes. The most commonly used system is the z-score, which... 

En numbers are used when the assigned value has been produced by a reference laboratory, which has provided an estimate of the expanded uncertainty. This scoring method also requires a valid estimate of the expanded uncertainty for each participant s result. A score of En < 1 is considered satisfactory. The acceptability criterion is different from that used for z-, z - or zeta-scores as En numbers are calculated using expanded uncertainties. However, the En number is equal to zeta/2 if a coverage factor of 2 is used to calculate the expanded uncertainties (see Chapter 6, Section 6.3.6). En numbers are not normally used by proficiency testing scheme providers but are often used in calibration studies. 

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. 

The target range for this proficiency testing scheme has been set at 0.03 %abv, based on fitness for purpose criteria. Using an assigned value (X) of 40.04 %abv... 

Making the Most of Participation in Proficiency Testing Schemes... 

Participation in proficiency testing schemes can bring significant benefits to laboratories. However, the proficiency testing scheme itself cannot cause improvements in laboratory performance. It is up to the participants to use the feedback they receive from the scheme to monitor their performance and to implement improvements where necessary. 

Participation in proficiency testing schemes is an ongoing activity. It is therefore useful to monitor performance over a period of time and to look for trends. Performance over time can be demonstrated statistically by using measures such as RSZ and SSZ (see Section 7.3.4) but as mentioned previously, these can be misleading. It is better to monitor performance scores by plotting them on a... 

The data shown in Table 7.3 are from one round of a proficiency testing scheme for the determination of moisture in barley. There are a number of different... 

This chapter has considered two of the types of interlaboratory comparison exercise in which your laboratory may participate. It is important to remember that proficiency testing schemes and collaborative studies have different aims. The former is a test of the performance of the laboratory, whereas the latter is used to evaluate the performance of a particular analytical method. Laboratories should participate in proficiency testing schemes (where an appropriate scheme is available) as this provides an independent check of the laboratory s performance. This chapter has described the key features of proficiency testing schemes and explained how the results from participation in a scheme should be interpreted. 

Proficiency Testing by Interlaboratory Comparisons - Part 1 Development and Operation of Proficiency Testing Schemes , ISO/IEC Guide 43-1 1997, International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC), Geneva, Switzerland, 1997. 

Guidelines for the Requirements for the Competence of Providers of Proficiency Testing Schemes , ILAC G13 2000, International Laboratory Accreditation Cooperation (ILAC), Silverwater, Australia, 2000. 

The nonconforming work that will require evidence of implementation of corrective and preventive action within a given timescale will include matters, such as, no corrective action taken when the results from a round of a Proficiency Testing scheme indicated the laboratory s result was an outlier, or the competency records of staff do not indicate they are competent to do the accredited work. Listings of nonconformities can be found in a publication produced by the International Laboratory Accreditation Cooperation (ILAC) [9]. 

Review of performance in laboratory intercomparisons or Proficiency Testing schemes... 

Where appropriate, performance in proficiency testing schemes and/or interlaboratory comparisons is satisfactory and has not highlighted any problems or potential problems. Where performance has been unsatisfactory, corrective action has been taken. 

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