Setting up a measurement study involves looking at the measurement system with respect to equipment, technique, environment, and the people involved. I’ll use an example to illustrate the approach. This example is a classic style of study and will examine a measurement study for assessing the system used to collect the pH of a product. For your first study, choose a measurement system that you are familiar with, and one where the tests are relatively quick and straightforward to perform.
The following steps are required to set up a basic study:
One piece of equipment that has been calibrated before the study. Make sure the equipment is in good working order.
Example: Use one pH meter. Perform a standard calibration before the study.
Two or three testers are required.
All testers should be fully trained in the test method. Ideally, they should complete the test regularly.
Testers should be unaware of the study, so the tests are conducted in the same way regular measurements are taken.
Ensure that the testers are given adequate time to perform the tests. Be sure that they do not become fatigued.
Example: Three testers are chosen. All use a pH meter as a normal part of their working day.
Ideally, 10 different samples should be used. This number can be reduced to as few as 3; however, a higher number of samples will provide more reliable 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 are assembled. One sample with a low pH, as acidic as would typically be seen for the test, has been selected. Another sample with a high pH, as alkaline as would normally be seen in the test, has also been selected. The other chosen samples will fall between these two extremes. Note: Sample selection is the same if parts are being measured rather than solutions.
Each sample is presented to each tester two or three times for testing. So, 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 distinguished for analysis but is not identifiable by the tester.
When using parts, each tester will measure the same parts two or three times, so the parts need to be identified in such a way that the testers will not realize that they are testing the same components more than once.
The sample’s actual value should be known, if possible. If an assessment is being made of a pH meter, then standard pH solutions can be used. Comparisons can then be made between the known answer and the results of the study. In some applications, known values are not always available.
Example: Three standard pH solutions are purchased for the study. Their known values are 3, 5, and 7. Three production samples with different pH values have also been selected. Since it is relatively easy to test standard samples, but can be more difficult to test product, both standard samples and product are 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 tests 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 is taken not to contaminate the sub-samples and alter the pH value. Each sub-sample has been identified with random numbers, and a note kept of each random number and the related standard solution or product.
Standard operating procedures should be used.
Present the samples to the testers in random order.
Example: The samples are presented to each tester in random order. Each tester tests all the samples and records the results on a table indicating the sample’s random number and the outcome.
It is essential when conducting a measurement study to avoid complexity. Keep the study as simple as possible. Otherwise, it can be challenging to ascertain meaningful information from the study data.
The next step is analyzing the data. I will address some basic analysis techniques in an upcoming blog post.