Background information

Background information - PTs, QC, method validation

Classification of interlaboratory studies

What are PTs? Interlaboratory studies? or ILS? Background information on quality control methods


Our work focuses on the evaluation and planning of interlaboratory studies (ILS). Here's a brief overview of the terminology surrounding interlaboratory studies:



The Basics

An interlaboratory study (or ring test) is one tool used in quality assurance. As the name suggests, the quality between comparable laboratories/institutes is checked.


The interlaboratory study provider sends similar samples or artifacts to the participants, who are then tasked with analyzing the samples in regard to one or more specific questions. The corresponding results are evaluated for quality of the measured values, participants’ performance, or the quality of the method.


Depending on the overall goal of the interlaboratory study, studies can differ in how they are conducted, the participants involved and in their evaluation. The most important types of interlaboratory studies are described briefly in below:


Proficiency Tests

A proficiency test is regarded as an external quality assurance tool.


The goal is to establish a regular and objective assessment of participants’ results, thereby providing feedback to the participants. The results grant the participants the option to identify errors in their practical work and the opportunity to correct such errors. The analysis between the participants allows for the assessment of the performance of each individual participant. According to ISO 17025, accredited testing and calibration laboratories must participate in proficiency tests on a regular basis in order to prove the quality of their measurement results and, in general, their competence.


Interlaboratory studies for proficiency testing are provided by e.g. accreditation bodies or specialized companies. The requirements for carrying out proficiency tests are regulated by ISO 17043. For example, one requirement is the sufficient homogeneity of the provided samples.


Statistical methods for assessing proficiency tests are described in ISO 13528. Herein, the computation of so-called “standardized performance statistics,” such as z or z’ scores, are described. These scores are the basis for assessing the measured results of the participants.



Method Validation Studies

It is necessary to validate standardized methods by interlaboratory studies. In such a method validation study method parameters are determined, including precision and trueness of the method. Not only is the variability of measured values within the results of one participant (repeatability) determined, but also the variability of measured values between the participants (reproducibility).


Classical method validation studies require, according to ISO 5725-2, at least eight participants. The factorial method validation study concept developed by QuoData only requires four participants – without any loss in the quality of method validation parameters.


The evaluation of method validation studies is described e.g. in ISO 5725 (parts 1 to 6).



Interlaboratory Studies Within One Company

Interlaboratory studies can also be conducted within one company. Here the results from different branches, methods or employees are checked to see if systematic measurement deviations or differences in precision are present. If so, correcting measures can be taken in order to improve quality within the company.



Interlaboratory Studies to Certify Reference Material:

Interlaboratory studies working with certified reference material (CRM) use their analytical results to assess the reference values for certified reference material. The quality of reference material is essential for the precision and reproducibility of analytical results.


The quality requirements for certified reference material are outlined in ISO guides 30 to 35.