Qualitative Testing and Root Cause Analysis

Qualitative Testing

Qualitative tests look for a change in a quality; for example, a color might change. Qualitative tests always involve the use of attributes rather than variables values (e.g., temperature). Consider the Kastle-Meyer test for the presence of blood–an archetype for qualitative forensic testing. The test is impressively quick and functions as a decision support tool by letting us know whether we move on to the much more involved, time-consuming, and expensive testing in a full-featured laboratory. What we see here is exactly what we want out of a good qualitative test![1]

Applied to Root Cause Analysis

This testing approach can help us understand probable areas of concern for additional investigation. Consider a poly-carbonate part that fails in a particular real world application.  A review of failed parts shows cracking or surface crazing.  In the field, the part develops fractures along mounting locations. It might be obvious to assume that the over torque of the screws may compromise the material.  It is also possible the mechanical stresses such as vibration contribute to the parts shortened life.  However, there can be other reasons for this same cracking.

Root Cause Analysis Exploration

The material data sheet shows a particular vulnerability to alcohols.  Not tequila or rum to which I am occasionally vulnerable, but the sort used in antifreeze and solvents for cleaning.  These sorts of materials can easily be found in the environment in which the product resides.  To understand the possibility this sort of chemical agent is contributory, a quick experiment is devised to expose the material to a consistent concentration for a brief time via vapor not immersion .  This amounts to some measure of accelerated testing as the material would not typically be exposed to this chemical for long periods of time.  Upon removing the part similar surface crazing is noticed similar to that of the failed field parts.  A hairline fracture on one of the mounting locations is also noticed very similar to the field failures.

With this simple quick test we have found another clue to further explore on our way to understanding the mechanisms of the failure. We show and more examples of testing in our book Testing of Complex and Embedded Systems.


[1] Pries, K., & Quigley, J. (2011). Overview. In Testing Complex and Embedded Systems (p. 18). Boca Raton, FL: CRC Press.

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