MSA: Setting up a measurement system

By Jackie Graham, Ph.D.

Setting up a measurement study requires a reminder that the measurement system includes equipment, technique, environment, and people involved. Each of these demands attention in establishing a sound study.

Let’s set up a simple study using an example to illustrate the approach. The example examines how to set up a measurement study for assessing the system used to collect the pH of a product. This is a classic style of study. For your very first study try to choose a measurement system with which you are familiar, and one where the tests are relatively simple and quick to perform. To set up a basic study the following are required:

  • Equipment
    One piece of equipment that has been calibrated prior to undertaking the study. Make sure the equipment is in good working order.

Example: use one pH meter. Perform a normal calibration prior to the study.

  • Testers
    Two or three testers are required.
    All testers should be fully trained in the test method. Ideally, they should complete the test on a regular basis.
    Ideally, the testers should be unaware of the study so the tests are conducted in the normal way.
    Ensure that the testers are given adequate time to perform the tests. Be sure that they do not become fatigued.

Example: three testers were chosen. All use a pH meter as a normal part of their working day.

  • Samples
    Ideally, 10 different samples should be used. This number can be reduced to as few as 3. However, the greater number of samples used, the more reliable the results.
    The samples should be different from one another and reflect the normal operating range of the equipment.

Example: for the pH assessment 6 samples were assembled. One sample with a low pH, as acidic as would normally be seen for the test, was selected. Another sample with a high pH, as alkaline as would normally be seen in the test, was also selected. The other samples were selected between these two extremes.

Sample selection is the same if parts are being measured rather than solutions.

Each sample is presented to each tester 2 or 3 times for testing. In the case of pH, each of the samples should be made as homogenous as possible and split into the required number of samples. If two testers are measuring each sample twice, this will be four samples. Each sample will need to be uniquely identified in such a way that it can be easily identified for analysis but is not identifiable by the tester.

When using parts, each tester will measure the same parts two or three times, so they need to be identified in such a way that the testers will not realize that they are testing the same parts more than once.

Ideally the sample’s true value should be known. If an assessment is being made of a pH meter, standard pH solutions can be used. Comparisons can then be made between the known answer and the results from the study. In some applications, known values are not always available.

Example: Three standard pH solutions were purchased for the study. Their known values were 3, 5, and 7.Three production samples with different pH values were also selected. Since it is relatively easy to test standard samples, but can be more difficult to test product, both standard samples and product were used to assess the measurement system. The standard pH solutions will reveal any bias issues, and the production samples will ensure that product can be reliably measured.

Each tester will test each sample twice, so six sub-samples are taken from each standard solution and six sub-samples from each of the production samples. Great care was taken not to contaminate the sub-samples and alter the pH value. Each sub-sample was identified with random numbers, and a note kept of each random number and the related standard solution or product.

  • Procedures
    Normal operating procedures should be used.
    Present the samples to the testers in a random order.

Example: the samples were presented to each tester in a random order. Each tester tested all the samples and recorded their results on a table indicating the sample’s random number and the result.

It is important when conducting a measurement study to avoid complexity. Keep the study as simple as possible. Otherwise, it can be difficult to ascertain meaningful information from the study data.

The next step is to analyse the data, so in the next article we will talk about some basic analysis techniques. We recommend GAGEpack for all your measurement system needs.

Originally published in the October 2007 edition of Quality eLine, our free monthly newletter.

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