Frequently Asked Questions

About PT selection and placing orders



Why should I like to participate in a Proficiency Test?
Participation in an iis Proficiency Test allows a laboratory to compare its own performance to those of other, similar laboratories. The presence of systematic errors and/or random errors will be revealed, which allows the laboratory to take - if it feels necessary - corrective actions to improve its performance. Furthermore, the results are compared to the requirements of internationally accepted standardized test methods (ISO, EN, ASTM or other industrial standards). Each laboratory gets a performance score (z-score) for each reported test result. Hopefully, this score - given by an independent and ISO/IEC17043 accredited institute (R007) - will prove the laboratory's good performance. Proof which can be used to convince your staff, your clients and your National Accreditation Body. See also other relevant information on this site: Proficiency Test and Statistics
How do I subscribe to an iis interlaboratory study?
There are many ways to enroll in our PT schemes. Most easy is to complete the form on the e-mail page and mail it us or send us a purchase order by mail or by fax. We will confirm your registration and in due time you will receive the respective samples.
Can I cancel my subscription?
Yes, you can cancel your subsciption until one week before the sample dispatch date. After that date already much of the costs are already made and therefore you will receive an invoice, even when you do not want us to send you the samples.
Are iis' Proficiency Tests recognised by the National Accreditation Bodies?
Yes. The quality system of iis is in full agreement with ISO/IEC 17043:2010. The ISO/IEC 17043 accreditation is granted by the Dutch Council for Accreditation ('Raad voor Accreditatie', RvA) since January 2000. Our registration number is R007, see the RvA website. Through the existing M.R.A.s also most other National Accreditation Boards do recognise the ISO/IEC 17043 accreditation of iis.
Is participation in Proficiency Tests mandatory for ISO 17025?
Regular participation to Proficiency Tests is prescribed in ISO/IEC 17025. National accreditation bodies (NAB) will audit whether your laboratory participates in relevant schemes and how the evaluation of the results is implemented in your quality system. The iis Proficiency Tests are presented to numerous NAB's.
Can I order Reference Materials from iis?
The Institute for Interlaboratory Studies is specialised in the organisation of interlaboratory studies and the development of Reference Materials (RM's). However, the scope of available RM's is limited and the RM's are only available for participants in one of the iis schemes. See also other relevant information on this site: Reference material Page
Does iis organise interlaboratory studies (Proficiency Tests) on request?
Yes. If you have special wishes, please get in contact to us. See also other relevant information on this site: Info on Proficiency Tests
How do I know that my registration order was received by iis?
All received registrations and orders (and all other received messages) will be confirmed either by fax or by e-mail. When you do not receive a message from us within 7 days, your registration order was NOT received by us!
Why is not each PT organized every year?
Not each laboratory is able to participate each year due to the high (transport) costs. Also, the importance for PT participation may differ significantly per laboratory and per product. Therefore the frequency of a number of PTs is limited to once every two years.
Why should a laboratory not try to get a perfect score in a PT? Is that not the goal?
NO! The goal is to get a good impression of the quality of the normal routine performance of the laboratory. And therefore no extra efforts should be used to reach a better performance than normal. No excessive testing, no extra quality control, no special treatment whatsoever should be used in the analysis of the PT samples. Only by treatment of the PT sample as if it is a routine sample, the quality of the laboratory can be evaluated correctly, and when necessary it can be improved after receipt of the PT report.
go to the top of this page
About costs and payment of invoices
What is the VAT number of iis?
The VAT number of iis is NL001855013B01.
When should I pay?
Invoices should be payed within 30 days after receipt of the invoice. Customers with a normal payment history do not need to pay in advance.
How should I pay?
We prefer bank credit transfers, but credit card payment is also possible.
Can I pay in US dollars?
Yes, but only when we are informed in advance so that we can prepare an invoice in US dollars or when you pay per credit card.
What are the bank details of iis?
See the 'Contact us' page for our bank details.
Why do I get invoices from SGS?
The name Institute for Interlaboratory Studies (iis) is a registered tradename of SGS Nederland BV. Therefore it is not possible to prepare invoices with iis letterhead. iis is a small, but fully independent department of the SGS Group, world's largest organisation in the field of inspection, testing, analysis and certification.
What are the payment terms?
Our invoices should be paid within 30 days after receipt. See also our General Terms and Conditions.
What happens when I pay late?
When your payment is not received with the payment terms, a standard minimum penalty payment for the collection costs (exclusive of interests and other costs) will be applicable (See Overview of costs on our Annual program page). This is in agreement with Dutch Law, effective since 13th December 2012.
Can I get a (stamped) hard copy of the invoice?
Yes, but when a hard copy of an invoice is sent, it will be sent by courier and consequently the DHL costs will be invoiced (See Overview of costs on our Annual program page).
go to the top of this page
About samples and sample transport
Why is the sample quantity limited?
As the results of our proficiency tests are used by accreditation bodies, we -as ISO/IEC17043 accredited organization- are not allowed to send a surplus of sample in order to avoid excessive testing to get good results. However, in some cases the total amount of available bulk material is very limited. Nevertheless, the sample amounts are always sufficient to perform all tests at least once.
Why are not all requested components present in the samples?
Our institute is accredited acc. to ISO/IEC 17043. In the accreditation documents is a requirement that tells us to use sample materials that are very realistic and like every day's routine samples. However as in practice hardly ever any forbidden components (e.g. AZO-dyes, metals or pesticides) are found in textile, this would mean that we should use samples without any contaminations at all. And that would not be suitable sample material in our opinion, because we cannot evaluate laboratory performance with only 'less than' results! Therefore we decided to use samples artificially contaminated with several components, and to use each round different components with different concentrations.  In this way the samples are like every day's routine samples for some components and at the same time give useful information for other components.
What can I do when the sample quantity is too small for my laboratory to perform all tests?
The participating laboratories do not have to perform all tests that are requested. Only tests that are performed routinely by the laboratory should be reported. And the laboratory may choose to perform only the tests that are most important for the laboratory.
Can I purchase extra sample(s)?
When a PT is still running it is not allowed to purchased extra samples. However, as soon as the PT is finished, it is indeed possible to order retained PT samples for example for training purposes or to investigate problems to determine corrective actions. All remaining PT samples are retained by iis under controlled conditions and made available for the participating laboratories to be purchased. The amount of remaining samples is limited and varies per product. Please contact iis for more information.
How can I receive the sample(s) as soon as possible after the start of the PT?
One has to understand that sample transport is a matter of pushing (by the sender) and pulling (by the recipient). Without the assistance of the recipient it is impossible to deliver a sample, no matter how hard this is tried by the sender. Therefore the (pro-active) assistance of the receiving company is of utmost importance in order to get the sample at the laboratory within an acceptable time frame. For example import licenses need to be arranged prior to the sample dispatch date.
What can I do when the samples arrived too late?
Please report to us about the delay and test the samples as usual. We will evaluate all reported results, also when reported after the deadline for submitting results.
What can I do when the samples arrived damaged?
Contact us as soon as possible. When a bottle has been broken, please do not accept the package from the courier. Please note that we will only send a replacement bottle at our cost when there is proof of the sample being broken upon receipt. This means that the consignee must report a broken bottle immediately. The consignee should take pictures of how the package and bottles looked upon arrival. We need these pictures to claim costs from the forwarders. The consignee should always mark any damage (dents, rips, leakage etc.) on the outside of the packages on the consignment note from the courier when it is delivered. Even minor damages should be reported. Please send copies of consignment notes and the pictures via email to
How can I avoid samples being delayed by customs?
Participants are responsible for the customs clearance of the samples. For some countries the receiver needs to have an import licence. The import license needs to be arranged prior to the sample dispatch date. If you require us to attach documents to your parcel, we need to receive the documents at least two weeks before the sample dispatch date.
go to the top of this page
About analytical testing and submission of results
  • What are the rules for testing a PT sample?
    A PT sample should be treated and tested exactly like any other sample that is tested by the participating laboratory, which is a requirement from ISO 17025 and ISO 17043. It should not receive any special treatment or attention. Excessive testing is not allowed. Only when the laboratory performs a test in duplicate routinely, the PT sample may also be tested in duplicate. No extra QC, nor extra calibration may be done that is deviating from the routine procedure that is used for regular samples to be tested. Also it is not allowed to contact other participating laboratories about the test results on the PT sample before the final PT report is published.
    What happens when the PT rules are violated?
    This will depend on who discovers the violation. When an external quality auditor finds the violation during a laboratory audit, without doubt a non-conformity will be raised and the certification or accreditation of the laboratory may be withdrawn. When an accreditation auditor finds the violation during the annual audit at iis, without doubt a non-conformity will be raised and iis will have to take extra measures. When iis finds the violation, the reported test results will not be used for evaluation and will not be published in the PT report. Also, the laboratory may be excluded from further participation in the iis' PT schemes.
    Can I report my results electronically?
    Yes. Since medio 2014, we have a web site to upload your test results: For test results on Consumer products please use For test results on Petroleum products, (petro)chemicals and gases, please use
    Where do I find the letter of instructions?
    The letter of instructions is sent to you by email on the sample dispatch date. In case you did not receive the email or otherwise lost the letter of instructions, you can always download it from the data entry website on Internet, see above.
    Can I report my results after deadline?
    Yes, you can report results after deadline.
    Of course we prefer that you will report results before deadline, but due to unforeseen circumstances it may happen that you are not able to report your results before deadline. After the official deadline you cannot enter your test results via the internet based data entry website. However, you can download the report form in Word format from the iis website. Please be assured that we will evaluate your results always after receipt, also when we receive your results long after deadline. We will send you a special report (usually one page for one sample) with our findings. It is not possible to have your late test results be published in the regular final report, nor is it possible to update the already published final report.
    Did iis receive my test results?
    Results that are received by fax are not confirmed by us. When you report via the website or you can verify yourself whether your test results were uploaded correctly. To upload your test results, you need to press the save button! However, when you report test results by e-mail, we will always confirm receipt of your results.
    Is it possible that multiple operators analyse the PT sample?
    Yes, multiple operators can test the PT sample, but only for different tests. The PT set up is not intended for different operators performing the same test and reporting multiple test results for this test. Therefore, test results from a second or third operator will not be evaluated. Such replicate test results will have to be evaluated by the laboratory itself. One has to keep in mind that a PT sample should be tested like any other regular sample, without special treatment like excessive testing.
    go to the top of this page
    About reports and statistics
    Why does iis not use Youden Plots?
    Youden plots are meant for a two sample design, using duplicate testing, whereas iis organises proficiency tests using a single sample design and reporting of single test results. The reason for this choice is our bad experiences with Youden plots. In some other interlaboratory studies the laboratories are asked to perform a duplicate analysis on one sample and the two results are considered a Youden pair, which is a wrong assumption. Furthermore the duplicates often are contaminated with falsified results. The only reason for this falsification is a psychological one! Any sound thinking person will think twice before reporting bad duplicate results and will perform a re-test before reporting. So, in practice the reported duplicates are much better matching than the original duplicates. When the statistician subsequently uses a Cochran outlier test to delete all duplicates that do not fit in the data set, the only truly reported duplicate values will be seen as outliers and rejected! The same may occur when two samples are sent to the laboratories and both individual results are used as Youden pair. Even when the sample set consists of blind replicates. Perhaps it may work once, but after the first final report, the participating labs know that duplicate samples are used and will treat all subsequent samples as duplicates. The only way to avoid this problem is to send more than 2 samples (e.g. 3,4,5....) and let the results/concentrations/values differ only marginally. In this way the labs will not know which samples are duplicates and which are not. Youden plots can also be made for samples that are not replicates, but with values that differ slightly! But this last option will give the labs a lot of work (testing at least 3 samples for the same tests!) and extra transport costs. So, there are some negative aspects to the use of Youden sample pairs. And as iis does not see many positive aspects (the only one we see is the determination of repeatability, but one does not need an interlaboratory study for that!), iis decided not to use Youden pairs and to save the labs a lot of work by testing only one sample.
    Why is iis using target z-score and not the usual z-score calculations in which standard deviation of the group of laboratories is used?
    This way of z-score calculation results in a simple, straight forward comparison of lab results with the reproducibility stated in the corresponding international accepted test method. It indicates how many times the standard deviation the reported result deviates from the 'true value'. Target z-scores allow evaluation of performance over longer periods using a series of PT-results. This is impossible with z-score calculated from the standard deviation of the group of participants as the group of participants is not constant, nor is its performance. This last version of the z-score ensures that about 95% of the test results will fall within limits of ±2 and only 0.5% will fall outside 3, this by definition and thus independent from the performance of the laboratories! See also R.G.Visser, W.Oussoren, Accred Qual Assur (1998)3:497-498 and/or Rob G. Visser, Accred Qual Assur (2006)10:521-526 . An independent article about the background and reasons for the choice of the use of an appropriate (target) standard deviation is given by Michael Thompson and Stephen Ellison (2006), Accred Qual Assur (2006)11:373-378.
    My recovery was OK, but my test result was evaluated to be bad?
    Yes, in some cases it may be possible that a test result with a good recovery (and thus close to the theoretical value) is considered to be a bad test result and therefore a large z-score is calculated for this test result. This will be the case when the vast majority of the laboratories has low recoveries, for example in the case of the determination of chromium 6+ in plastics or PFOA/PFOS in plastics. One has to realize that in iis PT's a consensus value is determined, based on the reported test results. Although PTs are designed to evaluate the comparibility of laboratory test results, in most cases the consensus value will be close to the 'true value', but in no case traceability to S.I. units is claimed.
    Why are the homogeneity test results not always in line with the PT consensus value?
    The average of the homogeneity test results indeed are not always in line with the average (consensus value) from the PT results. There are several reasons for this. First the goal of the homogeneity testing is very different from the goal of the evaluation of the reported PT results. In order to prove the homogeneity of the PT samples, a test method is selected with a high precision (smallest variation). The accuracy (trueness) of the test method is less relevant. Secondly, the homogeneity testing is done by one laboratory only. The test results of this (ISO/IEC 17025 accredited) laboratory will have a bias (systematic deviation) depending on the test method used. The desire to detect small variations between the PT samples leads to the use of a sensitive test method with high precision, which may be a test method with significant bias. Also each test result reported by the laboratories that participate in the PT will have a bias. However, some will have a positive bias and others a negative bias. These different biases compensate each other in the PT average (consensus value). Therefore the PT consensus value may deviate from the average of the homogeneity test. At the same time the accuracy of the PT consensus value is more reliable than the accuracy of the average of the results of the homogeneity test.
    What is the difference between a proficiency test and a collaborative trial?
    In a proficiency test the performance of the participating laboratories is evaluated, while in a collaborative trial the performance characteristics of a Standard Test Method are determined. Consequently in a proficiency test the laboratories use their usual test methods and the determination of a reliable assigned value is the ultimate goal, while in a collaborative trial only one test method is used - being the one under investigation - and the determination of the repeatability and the reproducibility is the ultimate goal.
    How can my laboratory show its good performance to clients?
    Certificates of Excellence and certificates of Participation are available as of October 1, 2004. Only when all results reported by the laboratory are within the target reproducibility limits a certificate of Excellence may be issued.
    Where can I find the iis protocol that describes the work flow of iis?
    The current version of the protocol can be downloaded here.
    go to the top of this page
    About all other matters
    Can others see how I performed in an iis Proficiency Test?
    The answer is simple: "NO!" iis handles all information supplied by the participating laboratories with great care and strictly confidential. No information is passed to third parties unless prior permission is received. The identity of individual participants is always maintained confidential and only known to a minimum of authorised iis-personnel. Furthermore, in all reports laboratories will only be represented by a code number. The Institute is aware of the fact that participants of proficiency test do not always wish to enclose their performance to third parties. The identity of a laboratory can only be made public by the laboratory itself.
    What is the most effective way to follow up my PT-results?
    In order to assist the participating laboratories getting maximum efficiency from their PT-participation, Rob Visser - manager of iis - wrote some guidelines that were published in an article in the international journal 'Accreditation and Quality Assurance'.
    go to the top of this page


    ©1996-2023 Institute for Interlaboratory Studies