Proficiency testing: an effective measure of competency of rubber characterization labs.

Test data are very important and vital in making technical and scientific decisions for commercial processes and technical operations. Tests are conducted using established test methods to generate data. The accuracy and validity of test data are an important quality characteristic of any characterization laboratory. A regular independent assessment of the technical performance of a laboratory is recommended as an important means of assuring the validity of measurements, and as a part of an overall quality strategy. A common approach to this assessment is the use of proficiency testing schemes. A Proficiency Testing (PT) scheme is a system for objectively evaluating laboratory results by external means, and includes regular comparison of a laboratory's results at intervals with those of other laboratories. This is achieved by regularly distributing homogeneous test samples to participating laboratories for analysis and reporting of the data. The main objective of a PT scheme is to help the participating laboratory to assess the accuracy of its test results. In addition, participation in an appropriate PT scheme is recommended for laboratories seeking accreditation.

Proficiency testing and its significance

Proficiency testing is a method of checking laboratory testing performance by means of an inter-laboratory test. Participation in PT schemes provides laboratories with an objective of assessing and demonstrating the reliability of data they are producing. Besides these, it is also mandatory for all accredited and applicant (for accreditation) laboratories to participate successfully in a proficiency testing program in their respective scope of testing.

Type of proficiency testing program

Proficiency testing techniques vary, depending on the nature of the item or material under test, test method in use and the number of testing laboratories participating. They possess the common feature of comparison of test results obtained by one testing laboratory with those obtained by other testing laboratories. In some programs, one of the participating laboratories may be controlling and coordinating.

The following are the major types of proficiency testing programs:

Inter-laboratory comparison

In this program, participating laboratories are provided with subsamples from a source of a suitable degree of homogeneity, which they are expected to test at comparable levels of competence. Features of such a program are:

* It is essential that the (sub) samples provided to each participant are sufficiently homogeneous so that any results later identified as extreme should not be attributed to any significant sample variability.

* The test results after completion are returned to the coordinating body and analyzed against an assigned value to give an indication of the performance of the individual laboratory and the group as a whole.

* This is the type commonly used by accrediting bodies when they conduct programs in the testing fields, and often involve chemical analysis.

* This type of proficiency testing is applied for food, water and other environmental materials.

Measurement comparison

These programs involve the measurement of a property of the same item or group of items circulated among the participating laboratories. Features of such a program are:

* The item will often be periodically returned to a central laboratory acting as a reference laboratory for testing, before being passed on to the next successive participating laboratory in order to determine whether any change has taken place to it or to its assigned reference values.

* The individual measurement results are compared with reference values established by the reference laboratory. The coordinator takes account of the claimed measurement uncertainty of each participating laboratory.

* This type of proficiency testing is applied for reference standards, gauges and instruments.

Split sample testing

This program involves samples of a product or a material being divided into two or more parts, with each participating laboratory testing one part of each sample.

* This program differs from inter-laboratory proficiency testing, as there is usually limited control of the homogeneity of the sample being divided.

* This technique is sometimes used by clients of laboratory services, including regulatory authorities.

* Such programs often need retention of sufficient material to resolve any perceived differences between the limited number of laboratories involved, by further analysis through additional laboratories.

* This type of proficiency testing program is used for the destructive type of testing program.

Proficiency testing program through inter-laboratory comparison

The purpose of inter-laboratory tests include:

* This type of test is used to determine the competency of an individual laboratory for specific tests (as per the requirement of the accreditation body).

* Inter-laboratory tests are effective in checking the consistency or comparing the competency of personnel involved in testing.

* This technique also provides confidence in the calibration of instruments.

* Inter-laboratory test results can be used to assign values to certified reference materials.

* It helps in ensuring the uniformity of test methods employed in different laboratories.

* Interlaboratory tests are conducted to achieve confidence in testing.

The benefits to the participating laboratory include:

* Provides reliability of data produced by the laboratory.

* Supplements the laboratories' own internal quality control checks, like calibration or tracebility of reference standard.

* Acts as a tool for validation of test method.

* Results of a proficiency testing program can be used for determination of measurement of uncertainty of a test method.

* Analysis and interpretation of PT test results helps in achieving uniformity of test results across the different laboratories.

The main activities involved to organize a PT program include:

* Selection of a PT program;

* inviting and getting acceptance from different testing laboratories for participation in a PT program;

* sample selection and preparation;

* ensuring homogeneity of the material;

* sample distribution to the participating laboratories with clear identification, instructions and test result format; and

* getting results from all participating laboratories and statistical analysis of data. All results are analyzed for "Z-score within" and "between laboratories" to assess the laboratory performance, explained subsequently.

* Preparation of report and feedback to the participating laboratories while maintaining confidentiality.

* Corrective actions, if any are required.

Calculation of Z-score

Z-score is a normalized value, which gives a "score" to each result relative to the other members in the group. So, a Z-score value close to zero means that the result agrees well with those from the other values.

The Classical Z-score: Z = (result-mean)/standard deviation 

The Robust Z-score: Z = (result-median)/normalized IQR 

Interquartile range (IQR) is the difference between the lower and upper quartiles. The lower quartile (Q1) is the value below which a quarter of the results lie. Similarly, the upper quartile (Q3) is the value above which a quarter of the results lie.

IQR = Q3 - Q1 Normalized interquartile range (NIQR): NIQR = IQR * 0.7413

Calculation of between lab. Z-score (ZB) and within lab. Z-score (ZW)

The between laboratory Z-score (ZB) is

ZB = (S - median [S]) / (IQR [S] x 0.7413)

S = (A + B)/[square root of (2)] = standardized sum of the two results for a laboratory (where A and B are results of two samples of the same test).

Within laboratory Z-score (ZW) is: ZW = (D- median [D]) / (IQR[D] X 0.7413)

D = (A - B)/[square root of (2)] = standardized difference between the two results for a laboratory (where A and B are results of two samples of the same test).

D = (A - B) /[square root of (2)] (If Med A > Med B) D = (B - A)/[square root of (2)] (If Med B > Med A)

Interpretation of results obtained from different laboratories using Z-score

Extent of departure from the median is expressed in terms of Z-factor for testing laboratories. If Z-factor exceeds the upper limit of +/-3, the laboratory is rated to be an outlier in the specific program and is immediately asked to identify the causes, take the corrective action and inform the accreditation body. For Z-scores:

* |Z| < 2 Satisfactory

* 2 [less than or equal to] |Z| < 3 Questionable

* |Z| > 3 Unsatisfactory

While testing two specimens, care must be taken during analyzing the data, and for a particular laboratory's performance both ZW and ZB should be considered simultaneously. Using only one Z-score may mislead, for example: Even in a case of higher bias (i.e., low reproducibility) it may give a low value for ZW, and similarly in a case of low precision (i.e., low repeatability) it may give a low value for ZB. For accessing a laboratory's technical competency, both ZW and ZB value should be low at the same time.

For calibration laboratories, [E.sub.n] ratio is expected to be less than |1| (absolute value). Where [E.sub.n] stands for error normalized and is defined as:

[E.sub.n] = (LAB - REF) / sq. rt. ([U.sup.2]LAB + [U.sup.2]REF) Where: LAB is a participating laboratory's result; [U.sub.LAB] is a participating laboratory's reported uncertainty; REF is a reference laboratory's result; and [U.sub.REF] is the reference laboratory's reported uncertainty.

Area of concern in designing a PT program

The main areas of concern in designing a PT program include:

* Suitability of the program for the laboratory;

* availability of test materials/artifacts;

* homogeneity of the sample;

* availability of standard reference material (SRM)/certified reference materials (CRM);

* sample stability/shelf life;

* design of formats/protocol;

* analysis of results;

* preparation of report;

* interpretation of results;

* follow-up action and feedback by participating laboratories; and

* ethical consideration.

Requirements of a laboratory for being a nodal agency or PT provider

The requirements of a laboratory for being a nodal agency or PT provider include:

* Expertise in the tests involved in the scheme:

* the ability to provide a homogenized material;

* the statistical expertise needed to evaluate the results;

* adequate knowledge in standards/specification;

* adequate competency to prepare PT report;

* to initiate / provide follow-up advice; and

* the ability to review the effectiveness of the scheme.

Conclusions

In the rubber industry, most of the decisions related to process problems, quality control, new development or any process modification are based on the result of quantitative analyses of different kinds. Hence, it is important to have some indication of the quality of this test result for the purpose in hand. It is also required to eliminate the replication of effort frequently expanded in obtaining them. Confidence in data obtained outside the user's own organization is a prerequisite to meeting this objective. In order to achieve this, it is now a requirement for laboratories to introduce quality assurance measures, such as use of validated methods of analysis, participation in a proficiency testing program and laboratory accreditation, to ensure that they are capable of and are pro viding data of the required quality.

Proficiency testing is gaining increasing importance as a quality assurance tool for laboratories. The performance of laboratories in proficiency testing schemes is also being increasingly used, particularly by accreditation bodies, as a measure of the competence and quality of laboratories. It is important for laboratories to have comprehensive information on the scope and availability of proficiency testing schemes in the areas in which they work. This will enable them to make appropriate decisions about in which scheme they should participate. Laboratories, therefore, need to develop a good working understanding of proficiency testing, what the objectives of proficiency testing are, and how the data from proficiency testing schemes should be evaluated and used. This is important not only for laboratory staff and management within laboratories, but also for those who use their results, including accreditation bodies and the laboratory's customers.

References

1. Proficiency testing program for testing and calibration laboratories (NABL Procedure; NABL-162).

2. Policies and procedures for inter-laboratory comparisons and/or proficiency testing (NABL Procedure; NABL-163).

3. Course material of "National workshop on proficiency testing." conducted by Confederation of Indian Industries, March, 2001.

4. Eurachem Guide on Selection, Use and Interpretation of PT Schemes.

5. Vivek Bhandari, Arup K. Chandra and P.K. Mohamed, "Judgement of competency of rubber characterization laboratory through proficiency testing," paper presented at Rubber Expo-2005, Mumbai, India.