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ISO 17025 FAQ Questions, and PQA's Answer

Submit a Question for ISO 17025 FAQ. Send your question by email to dwhitred@pqa.net

Q1: Inter-Lab Comparisons
Q2: What does it take for ISO 17025 Certification
Q3: Measurement Accuracy & Units Conversion 

Q1: Interlab Comparisons

I have some queries regarding Inter lab comparison.

Firstly, when I'm the originator and participating lab at the same time, how can I ensure that the results obtained are correct? Is it necessary that I should know the actual results before analysis? I mean if I'm getting 15% result of x sample and the other lab is getting 14.8% result, whereas the actual value should be around 20%, how would I interpret the results? Because I would judge the both results, and if they are close to each other, I will consider it as the final result. Therefore according to me, in this scenario, 14.9% would be the final value.

Also what is the outcome of this activity? I mean after analysis and gathering results from all labs, what should be my next step?

Waiting for your reply


Amna Malik Quality Assurance Officer


PQA's Answer

For inter-lab comparison, you are attempting to determine:

  • the magnitude and trend over time in any errors;
  • factors that are affecting the results (ie. age of reagents, lab temperature, lab technician, etc.)
  • the inter-lab calibration factors for mean and variance, and comparison to the true value

Before starting your analysis, lets assume your lab technician knows that the real sample value (ie. as tested by a National Laboratory is 16.285), and all other labs have already tested & submitted their results as 16.284, 16.286, 16.285, and 16.287  Do you think the lab tech will report some drastically different value?  I don't think so.  If they get a very different value, they will search forever to find their error, or secretly re-test to come up with the same answer as everybody else. Will they do the same for an unknown customer's sample?  I don't think so.  This prior knowledge defeats the purpose of the inter-lab tests.  The lab technician must not know the true sample value before analyzing the sample, nor the results of others.  This ensures their independence.  Get someone else trained (other than the person testing your sample) to prepare your samples, collect the data from other labs, and secretly label the samples so no one (except the person holding the secret key) can tell them apart. When you get the results from everybody, are some labs consistently higher, others consistently low?  If a lab tests the same sample divided into 10 equal parts, do they get the same answer for all 10 samples? If you take 10 different samples randomly spread over the normal operating range of the customer's samples, divide those samples into two (making 20 samples, 10 pairs), when labeled randomly so tester cannot tell which sample is which (but you secretly know), can the tester tell which of the 20 samples are pairs?  This experiment has replication, so you can estimate the error of the testing method. The data must be analyzed statistically to determine the error due to the lab technician, error due to the test method itself, and random error.

Using DOE (Design of Experiments) will maximize the knowledge gained with the minimum number of samples tested.  If your test costs (eg. lab tech salary, reagents, sample preparation time, sample cost, etc.) are significant/expensive, you economically MUST use DOE.  If you don't have the skills, learn them, or get somebody hired/contracted who does. PQA can perform this statistical analysis for you, or you can buy the software, take a course, and do it for yourself. Otherwise, you can hire someone locally.  But somebody must do it. One simple way to see what you have is to plot true sample value on the Y axis, the lab's result on the X axis, and a different data point style (ie. circle, square, etc.) for each lab.  Of course, all data points should lie on a 45 degree line, but this rarely occurs due to intra- and inter-lab errors.

If everything is fine today, how does it change over time?  Summer vs. winter?  New lab tech vs. experienced veteran?  The new test machine vs. the old reliable machine? Once you have the statistical analysis done, you can calculate the calibration factor for each participating lab, lab technician, analysis machine, etc. One way of doing it (it may not be best for you) is to report the true value, as calculated by the calibration factors, so that no matter how & where the test is done, you know the true value, and this true value is what is reported to the client.  This takes a lot of data, tests, and effort on an on-going basis, to ensure the calibration stays consistent.

Glenn Black   P.Eng.   CQE   CQA

Q2: What does it take for ISO 17025 Certification

Good evening, I am interested in finding out what it takes to become an ISO 17025 Certified Laboratory.

Allen Hardison, Los Angeles California, USA


PQA's Answer

I quickly toured your website.  I assume your inquiry is related to _________, and you are planning a lab, or getting certified for an existing lab. Our role is to help you get ready for 17025 certification as quickly and cheaply as possible.  Most people waste precious resources in the certification as it's their first time.  After you have done as many as us, you get good and fast, without cutting corners.  If fact, you get optimum implementation that helps your operation improve its competitiveness, rather than barely meeting the minimum requirements.  Find someone locally with similar outlook and experience, or hire PQA. We would start by doing an on-site audit, learn what you need, then design a custom implementation strategy.  We then help you get started, getting you over all the hard items before we leave. This would take at least 2 days on site, to as much as 5 days, depending on your circumstances.  After that, you get ongoing support for whatever you need via phone, fax, email, and Internet until you get your certification. We can help you do an Internal Auditing training course while we are onsite, or you can do it later on-line for our website.  Either way, you can take care of your system as you design and implement it. You can get certified in 4 weeks to 6 months, depending on where you're starting from. To discuss further, give me a call

Glenn Black   P.Eng.   CQE   CQA


Q3: Measurement Accuracy & Unit Conversion

Don't know if you can help me out here, but here goes...

I am working on my Master's thesis, and I am confused as to how to proceed here. I have read a number of citations on rounding error/sig. digits, but I am still at a loss at to how to proceed. Perhaps you could point me towards some reference I could use. Here is my problem. I have measured the inside bend radius of my test samples using a set of radius gauges. The gauges are incremented in 1/64" steps, so it's a case of 'best fit' to a particular gauge. All measurements were either 7/64", 1/8", or 9/64". Now, I have to present these data in decimal inches and in SI, so 1/8" would become 0.125". But, converting 7/64 or 9/64 using the same conversion factor gives 6 decimal places before resolving to zeros, and keeping that many figures seems wrong (One publication I read stated that unit conversions should retain all figures, or something to that effect). Also, I have to convert to SI, so again, I have a question as to how many digits to retain. A more fundamental issue (I think) is that I can only resolve 1/64", so wouldn't my measurement for 1/8" be 1/8" plus or minus 1/128"? And again, how would one properly represent these measurements in SI. It's kind of frustrating, because 1), I feel I should know this, and 2) I feel I should be able to find some reference that discussed these issues. So, I'm hoping you have the time to point me in the right direction. Maybe I'm not framing the question properly in the search engines I am using.

Many thanks,

Vic Lewis

PQA's Answer

They are certainly good questions, and many people get stumped on these same issues. In conversion, there are a few schools of thought.

  • There is enough error in the original measurement.  You don't need to introduce additional error by units conversion.
  • To be reversible, you need a sufficient number of digits.  For example to go from Imperial to SI and back again to Imperial, can you regain the same original value?  How many significant figures do you have to carry to retain the accuracy of the original measurement in this cyclic conversion?
  • As your gauge is in ranges, the extreme measurements included in this bracket can be converted to SI with maximum digits retained, then take the median, then round off to the level of the original Imperial measurement. If the measurement gauge increment is 1/64", you can interpolate to +1/128"  (half an increment, or + 0.007813) as you stated.  Which means you really know the true usually within 1/3 to 1/10 when we allow for all the other sources of error (need R&R study to know for sure), so your true sample size is + 0.023438 or as close as +0.07813.  The number of significant digits should be able to resolve to this level. If all samples measured from 7/64" to 9/64" you have a range of 1/32",  so you should have a measurement gauge good to 1/320" (ie. can repeatedly differentiate + 1/320" with a 95% probability, including the person eyeballing the samples, so as to get an Error of Measurement  < 10% of the process variation.  Good luck on that.  To me, this is the largest problem, and everything else is a moot point, because garbage in, garbage out.
Submit a Question for ISO 17025 FAQ. Send your question by email to dwhitred@pqa.net

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