The terms quality and reliability are often used
interchangeably, but while they are strongly connected, they are
not the same. While reliability is concerned with the
performance of a product over its entire lifetime, quality is
concerned with the performance of a product at one point in
time, usually at the end of the manufacturing phase, to assure
conformance to specifications. Reliability assures that
components, equipment and systems function without failure for
desired periods during their whole design life .
One can say that reliability can be perceived as the
continuation of quality over time. However, the underlying goal
of quality and reliability systems is the same: to achieve
customer satisfaction .
Design for Six Sigma (DFSS) has been widely
employed as the framework to ensure that an organizations
quality objectives are met and Design for Reliability (DFR) is
becoming increasingly accepted as the best practice approach for
ensuring reliability within the design. The two processes are
complementary and there are many correlations between the tools
employed by each. In this article, we present a brief summary of
a comprehensive Design for Reliability process and we explore
the relationship between two tools that are employed in the
early product definition stage: the House of Quality (HoQ),
which is part of the Quality Function Deployment (QFD)
techniques used in Design for Six Sigma and the FMEA, which is
part of the Design for Reliability process.
DFSS processes are widely known within industry,
so in this article we will assume that the reader has adequate
exposure in that area. We will just focus on providing an
overview of DFR process in order to familiarize the reader with
this framework before proceeding to discuss two specific tools
in more detail.
What is Design
for Reliability (DFR)?
Design for Reliability refers to the process of designing
reliability into products in order to ensure that customer
expectations for reliability are fully met while minimizing
costs and increasing profit margins. In simple terms, whereas
individual reliability analysis methods enable the computation
of the reliability of an item, Design for Reliability provides a
process for assuring that the optimum/desired reliability is
designed into the item. This process encompasses multiple tools
and practices in order to drive reliability into products.
The three basic underlying concepts that should be kept in mind
under a DFR framework are:
Reliability must be designed into products
and processes, using the best available science-based
Knowing how to calculate reliability is
important, but knowing how to achieve reliability is
equally, if not more, important.
Design for Reliability practices must begin
early in the design process and must be well integrated into
the overall product development cycle.
The main DFR strategies are to:
Design out failure mechanisms.
Reduce variation in product strength.
Reduce the effect of usage/environment.
Increase design margins.
ReliaSoft has proposed a comprehensive DFR process in order
to enable organizations to achieve their reliability goals.
It is summarized in the following chart: *
[Click to Enlarge]
Six Sigma QFD and the House of Quality
In the world of quality and Six Sigma, Quality Function
Deployment (QFD) techniques are widely known and used. QFD
is a methodology that systematically translates the "voice
of the customer" (VOC) into functional requirements for the
product. QFD helps transform customer needs into engineering
characteristics for a product or service, prioritizing each
product or service characteristic while simultaneously
setting development targets for a product or service.
Through a planning matrix named the "House of Quality"
(HoQ), QFD transforms customers wants into designs,
manufacturing processes and production control requirements.
The transformation of customer requirements will help
develop part characteristics, process requirements and
product standards necessary for the product development
phase [3, 4].
In Figure 1, we illustrate a simple base structure for the
House of Quality . Keep in mind that
there is no "standard" format for the HoQ, because it can
and should be customized to serve the specific application
needs. It would be different for mapping out a new product,
redesigning an existing product or improving an existing
service. But the underlying concept is always mapping
customer requirements through the voice of the customer
(VOC) to technical requirements; in other words, matching
the "whats" to the "hows."
Figure 1: The House of
Modes and Effects Analysis (FMEA)
FMEA is an analytical engineering technique to ensure that
all the potential failures of a product have been considered
and analyzed in terms of failure modes, related causes and
possible effects on the customer. The FMEA may be employed
at various stages of the DFR process and may also be
incorporated into DFSS and other activities as well.
The FMEA inputs can be briefly demonstrated through a simple
example for the tires in a vehicle:
Function: Maintain the proper level
of inflation (e.g. 30 psi in front and 29 psi in
Failure: Insufficient inflation (flat
Effects: Inability to drive, damage
to rims, loss of control, etc.
Causes: Puncture, worn sides,
improper maintenance, etc.
Controls: Check tire pressure when
filling tank, rotate and replace tires according to
manufacturers guidelines, etc.
The key to a successful FMEA is to be "risk conscious" and
to keep the team focused on risk. Two commonly used
approaches for risk assessment with the FMEA framework are
Risk Priority Numbers (Severity x Occurrence x Detection)
and Criticality Analysis.
Between the House of Quality and FMEA
It can be said that the House of Quality (as a part of a
DFSS process) and the FMEA (as part of a DFR process) are
essentially two technical tools dealing with the same issue,
namely the customers satisfaction, with each operating from
a different point of view. QFD can be viewed as the defender
of the customer needs, while FMEA is the defender of the
voice of the engineer .
As quality is deployed into the design through the use of
tools such as the House of Quality, at the same time the DFR
process begins with input from the HoQ. Through the HoQ,
functional specifications are fed into the FMEA, which can
then be used to identify design risks and evaluate whether
functional requirements can be met. At the same time, the
results of the FMEA can be fed back into the HoQ in order to
close the gaps between the customer requirements and the
achievable reliability characteristics that will enable the
specific product attributes to be matched to product
requirements. Identified design improvements are also
reflected back to the technical requirements.
In conclusion, it can be stated that the two tools are
complementary, and when used in sync can be very beneficial
to map out product requirements and close the gap with the
reliability characteristics of a new product.
 Madu, C. N., "Reliability and Quality
International Journal of Quality & Reliability Management
vol. 16, no. 7, pp. 691 698, 1999.
 ReliaSoft Publications,
"Design for Reliability: Overview of the Process and
Applicable Techniques", Reliability Edge, Volume
8, Issue 2.
 Braglia, M., Fantoni, G. and Frosolini,
M., "The House of Reliability," International Journal of
Quality and Reliability Management vol. 24, no. 4, pp.
 Wolfe, P., Planning for Failure: HOQ
vs. FMEA, QFD Online,
 Benbow, D. W., and Kubiak, T. M., The
Certified Six Sigma Black Belt Handbook, Milwaukee, WI:
ASQ Quality Press, 2005.
*If you want to learn more about DFR,
ReliaSoft offers the training course
D560 - Design for Reliability (DFR) Program Planning and Implementation, as
part of the company's training curriculum. This course
provides the length and breadth of the most effective Design
for Reliability (DFR) methods, including an overview of the
tools that support DFR. Also included is instruction in
developing a successful DFR program for your company or