Measuring Softness, by Certified Softness
We measure the perceived softness (or handfeel) of objects and materials, using human touch. We report the results from each measurement as a numerical softness value, on a linear scale.
Linear Scale
Our measurement methods produce softness values that are on a linear scale. This means that 10 points of difference in any area of the scale represents the same perceived magnitude of difference in softness as 10 points of difference anywhere else on the scale. It is a somewhat arbitrary scale, like Fahrenheit and Celsius were for temperature before absolute zero was known.
Softness Test Methods We Use and Teach
The first two test methods above were developed at Scott Paper Company, and used worldwide. The Two-Out-of-Five Method is a textbook method, also used by Scott Paper. The Advanced Methods were developed by Carl Ingalls.
Control Method
The Control Method is used for process control and for routine quality assurance. It is a good choice when:
- speed and efficiency is more important than accuracy
- test samples do not vary greatly in the way they were manufactured
- a highly skilled and experienced assessor is doing the testing
- accurate reference standards are available and in sufficient quantity
- test samples are similar to reference standards
We normally use one or two experienced assessors for the Control Method. The process is a series of paired comparisons, where test samples are compared only against reference standards that have known values.
Disadvantages:
- low precision
- requires accurate reference standards (where the Panel Method is used to determine softness values)
Panel Method
The Panel Method is used for competitive analysis, product development, supporting advertising claims, creating a linear softness scale, certifying reference standards, or whenever higher accuracy is required. It is a good choice when:
- accuracy is more important than speed
- the test includes samples that were made very differently
- samples need to be compared with each other
- evaluating individual abilities to compare softness
- adding reference standards
We use a paired comparison sensory panel method that has been adapted for the perception of softness. A “Panel” is a set of samples to be measured (which may or may not include reference standards) evaluated by a group of people that we call “Panelists”.
We use a mix of experienced and inexperienced panelists who evaluate the softness of samples while touching the samples with their hands and following a protocol. Panelists feel two samples at a time, and they respond by telling us which sample is softer, and by how much. In some protocols, panelists are allowed to see the samples. Other protocols call for “blind” testing.
We calculate the consistency of each panelist, and use that to select which ones are to be included in the overall panel results and which are to be excluded.
Consistency means a combination of:
- self-consistency
- consistency with the other panelists
- consistency with the reference standards (when present)
Accuracy in the final results depends on:
- number of panelists
- skill of the panelists
- number and consistency of samples
- number of times each sample is compared with a different sample
- quality, consistency, and choice of reference standards
- which protocol is used
Analysis of the pair comparison data is based on standard statistical methods.
Disadvantages:
- Time and effort - a lot of samples to prepare and a lot of comparisons to obtain and record
- Data processing is difficult to do by hand (normally requires a computer)
- Limited to 9 samples total per panel, including reference standards
Two-Out-of-Five Method
The Two-Out-of-Five Method is used for determining whether there is a detectable and repeatable difference in feel between two samples. It is a good choice when:
- requiring a test that is very simple to do, easy to learn, and easy to understand
- making sure that two batches of samples feel the same
- demonstrating differences to a potential customer
The test uses 2 identical specimens from Sample A and 3 identical specimens from Sample B. Each specimen is labeled with its sample letter (A or B). All 5 specimens are arranged in random order, and presented together to the assessor. The assessor is not allowed to see the specimens, only touch them.
The assessor is told that there are two groups of specimens mixed together, 2 in one group and 3 in the other, and is instructed to separate them by feel into separate piles. The probability that the assessor could arrange the specimens as AA in one pile and BBB in the other only by chance is 1 out of 10. If several assessors do this, then we conclude that there is a detectable and repeatable difference.
Disadvantages:
- assessors may respond to any difference in feel, not just softness
- does not provide a numerical softness value
- does not provide magnitude of difference
Advanced Methods
We have developed variations and combinations of the test methods described above, and we teach the ones that are most appropriate for each client. These advanced methods are good choices when:
- comparing 10 or more samples with each other
- requiring more accuracy than the Control Method
Disadvantages:
- more complicated (harder to learn, harder to understand)
- additional training may be required