In the article you state “Various fixes have been applied to the severity rating to reflect its ordinal nature.  Richard Harpster defines the “legal zone” to include any failure mode with a 9 or 10 severity in “How to Get More Out of your FMEAs”.

The statement misrepresents what the article actually said.  The article states “Those who use FMEAs need to learn that the class value is more important than the RPN.  The class value is derived from the severity/occurrence matrix shown in Figure 2 below, puts the RPN into its proper perspective.”  When I wrote the article in 1999, the AIAG FMEA Manual 2nd Version was in effect.  In the manual, the occurrence rating of a 1 indicated a failure of at least 1 in 1.5 million.  If a company was designing automobiles and 1 out of every 1.5 million of their customers was going to be severely injured or die due to a known design issue, I believe most people would identify this combination of severity of harm and probability of exposure to indicate an unacceptable level of risk.  If one were to look at it from a financial perspective, I am quite sure the financial penalties would be quite high in a court of law if it came out in testimony that an automotive company was knowingly shipping vehicles with a design flaw that they believed would result in the death or injury at a probability of 1 in 1.5 million.  Consequently, given the limitation of using the Occurrence Rating Table in the AIAG FMEA Manual 2nd Version, the paper put the combination of Severity of 9 or 10 and Occurrence of 1 in the “legal zone” indicating unacceptable risk.  If you had included a copy of the entire matrix from the 1999 article people would be able to see the important relationship that the article defined between severity of harm, probability of exposure to harm and risk.

One of the most important improvements in the AIAG 4th Edition FMEA manual was the changing of the definition of the Occurrence Rating of 1 in the Design and Process FMEA from a numerical value of “</= .01 per 1 thousand” to the statement that “The failure is eliminated through prevention control”.  This allowed the creator of the Design and/or Process FMEA to properly identify the level of harm as a Safety or Regulatory Issue while assigning an occurrence rating of 1 that indicated that despite the high level of harm the risk is acceptable because of the actions the company had taken to reduce the probability of exposure.

One should never just use the Severity Rating to make any decisions to determine what must be worked on in any Design or Process FMEA.

Richard Harpster

When managing risk, it is critical to consider the products intended use.  As an example, I have recently worked on both spinal implants and a late stage cancer treatment.  In both cases, there were five categories of harm considered: Death, Permanent Injury, Injury Requiring Medical Attention, Injury Not Requiring Medical Attention and Inconvenience or Temporary Discomfort. Because of the differences in intended use, the definition of acceptable risk was different.  Although attempts were made to reduce all risk, the definition of acceptable risk was much higher for the late stage cancer treatment than for the spinal implant.  

The combination of the Severity/Occurrence Risk Matrix and Intended Use should be used to determine what must be worked on and acceptable levels of risk.  The resultant strategy is known as a company's "Risk Policy" for the product.

Bill; you have done some extensive research into the different approaches to risk for this article. I can tell you put a lot of work into providing your readers with good information. Thank you.