April 29th, 2008 by Peter Wolfe
You’ve probably heard the old adage, “If you fail to plan, then you’re planning to fail.” That sentiment is certainly echoed in the basic principles of the Quality Function Deployment (QFD) methodology. There is another old adage concerning failure that, although not quite as recognized, is just as true: “Fail to mitigate failure and you will succeed in minimizing success.” (Okay, so it isn’t really an old adage. I just made it up. However, you have to admit, it does sound rather catchy, and it does convey the underlying precept fairly well.) This maxim (regardless of how it is worded) is similarly echoed by the tenets of Quality Function Deployment.
So then, the question arises–what is the best tool for prioritizing steps to mitigate potential failures: Failure Mode and Effects Analysis (FMEA) or the House of Quality (HOQ) tool? Coming from a Quality Function Deployment enthusiast, my answer may surprise you…
Planning for Affects vs. Planning for Effects
The House of Quality is a fantastic tool. It helps to capture, quantify, categorize, and prioritize the “Voice of the Customer”. That being said, it does have its place and its purpose, and that purpose is primarily for planning the activities required to augment and improve a particular product or service. In other words, it is designed to assist in planning how a team should affect or modify the product or service in question. It really wasn’t created for the intent of analyzing the effects of failures or breakdowns and prioritizing the need to address those different failure modes.
A House-shaped Peg in a Square Hole
I have heard of teams trying to use a House of Quality for the purpose of mitigating failure. There are two major problems with taking such a path: First the mathematical structure of an HOQ doesn’t lend itself well to prioritizing activities for mitigating failure. Secondly, the HOQ doesn’t have fields for identifying and analyzing the causes, effects, and controls related to different potential failure modes.
Failure Mode and Effects Analysis on the other hand was created specifically for the purpose of analyzing potential failure modes and prioritizing activities to mitigate these failures. Thus, its mathematical structure is better suited for such a task. Additionally, it contains fields that guide its users through the analysis of different potential defects and their impact on one’s customers.
No Additives, Just Preservatives
As far as the math goes, one should note that the House of Quality is setup on an “additive” scale. If a particular functional requirement satisfies two separate customer requirements, then the weightings of those two customer requirements are added together (after multiplying them by their respective rating factor). Note that those weightings are not multiplied by each other.
This use of addition rather than multiplication makes sense–a characteristic that satisfies two requirements is probably only as important as the sum of the importance of the individual requirements. It does not become exponentially more important simply because it satisfies multiple requirements. Consider the following example:
Requirement A is weighted by a customer to be just as important as Requirement B. If a particular characteristic satisfies both A and B, and satisfies each requirement to the same degree, then it is reasonable to say that is twice as important as a characteristic that just satisfies either requirement. However, it would be unreasonable to say that it is five or ten times more important than a characteristic that satisfies only A or B.
The math for an FMEA, on the other hand, is essentially multiplicative in nature. The probability of occurrence rating is multiplied by the severity rating, which is multiplied by the probability of escaped detection rating. This too makes sense when considered properly–an error that will happen almost universally with catastrophic consequences is far more than twice as disconcerting as a catastrophic error that will rarely if ever happen. Similarly, such a potential failure is far more than twice as dangerous as a defect that happens universally with minimal effects. Thus, in an FMEA, the Risk Priority Number (RPN) is rightfully a calculation of the different ratings multiplied times each other.
Step By Step
As mentioned earlier, an HOQ does not contain any fields specifically designed to walk its users through an analysis of the potential failures for a given product or service. An FMEA, on the other hand, does contain such fields, and it is in the filling out of such fields that discovery and realization often takes place for an engineering or management team. An FMEA forces its participants to not only consider a given failure mode, but to consider the effects and causes of those failures. Additionally, an FMEA encourages its users to consider any controls that might already be in place for preventing or mitigating such a failure. This step-by-step analysis helps teams to properly consider the different aspects of a potential failure so that they can properly rate it. Without such analysis and/or information gathering, team members might be prone to rashly rate the priority of addressing a potential breakdown.
Right Tool for the Right Job
In summary, although the House of Quality is a fantastic tool for capturing and quantifying the Voice of the Customer, it is not the tool of choice for prioritizing efforts to prevent failures, defects, and/or breakdowns. Failure Mode and Effects Analysis, on the other hand, is an excellent tool for prioritizing efforts to mitigate failure. In short, HOQ’s are excellent for mapping Critical to Customer requirements (CTC’s) to Critical to Quality parameters (CTQ’s). However, FMEA is a better tool for quantifying the effects of failing to meet those CTQ’s.
Side Note: QFD the Tool vs. QFD the Methodology
Because the term “Quality Function Deployment” has come to mean two largely different things to the quality community, I should clarify something…At the beginning of this article I mentioned that my recommendations on the use of FMEA vs. HOQ’s might surprise people because I am a proponent of QFD. In truth, you may or may not be surprised depending on what the term “QFD” means to you.
Depending on where you first became exposed to the term you may think that it is either synonymous with the House of Quality tool, or you may think that the House of Quality tool is simply a minor subset of the QFD methodology. In actuality, QFD was originally (and still is) a full methodology that simply utilized the House of Quality matrix as one of its tools. However, other methodologies (such as Design for Six Sigma) have come to label the series of sequential House of Quality matrices as a “QFD“. (For more information on this topic, take a look at the article “Where Did QFD Get Its Terrible Name?”.)
If you are used to referring to QFD as a tool, then my opinions recommendation to use FMEA for failure mitigation may surprise you. However, if you are acquainted with the original Quality Function Deployment methodology, then my comments are probably quite unremarkable since the QFD methodology generally incorporates FMEA as an integral step.
This entry was posted on Tuesday, April 29th, 2008 at 12:00 pm and is filed under History of QFD, House of Quality, Advice, Voice of the Customer, DFSS, Quality Function Deployment, QFD, CTQ, CTC, FMEA. You can follow any responses to this entry through the RSS 2.0 feed. You can also leave a response.