One of the most important aspects of any quality system is the control of monitoring and measurement processes. The systematic and random errors that can influence test results or process monitoring results can be from a variety of sources. It is important to identify the cause and effect relationships that can create the variability observed in test results. Generally, these sources can be summarized into the following categories:

Equipment

The equipment, whether it is a sophisticated automated electronic analyzer or glassware, has been manufactured to certain tolerances. The variability inherent to the equipment specifications will be reflected in the test results. Component wear, failure, or inadequate maintenance will increase the inconsistencies in test results. Any inconsistencies in calibration verification and/or recalibration will also affect the consistency of the results obtained from the equipment.

People

People are almost always a contributor to some of these differences in results simply because none of us are exactly alike. We differ in dexterity, reaction times, color sensitivity, and other ways. Even the same operators can perform differently at different times due to different degrees of mental and physical alertness. Some operator differences are practically unavoidable. Of course, some tests are more sensitive to the effects of operator differences. Incomplete or confusing test methods or procedures open the door to another difference in operators, “interpretation” of the requirements or methodology.

Laboratory Environment

Some samples and equipment may be susceptible to temperature, humidity, atmospheric pressure, and other environmental factors. Because these cannot be controlled perfectly within or between labs, they provide some contribution to the variation of test results.

Samples

Any non-uniformity of the sample can cause additional differences in test results. When conducting studies to determine testing variability, special effort must be made to obtain test samples that are as uniform or similar as possible.

Time

All of the previously mentioned sources of variation can themselves change with time. In measurement studies, efforts are usually made to keep the time span as short as practical.

When taken together these many sources can cause significant issues with the repeatability and reproducibility of a measurement process. It is essential that steps be taken to minimize the differences in test results obtained within a given laboratory or Quality Control area. Working to reduce the effects of any or all of these sources can improve the consistency of your results and build confidence in your quality system.

Davis M. Woodruff, PE, CMC is an internationally recognized consultant, professional speaker and author who is an expert in showing companies how to be the low cost, high quality, environmentally responsible leader in their industry. The benefits he brings to his clients include: developing leaders; optimizing resource utilization; improving processes, quality and customer satisfaction; and saving time and $$$. Since 1984 he has served clients in 35 states and on 3 continents. Davis is the author of a full length book Taking Care of the Basics: 101 Success Factors for Managers, and dozens of published articles. He is a 1972 Engineering graduate of Auburn University, a Certified Management Consultant (CMC) and Professional Engineer (PE). His consulting firm, Management Methods based in Decatur, AL, is now in its third decade. Davis can be reached at davisw@managementmethods.com or for more information visit http://www.daviswoodruff.com

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