From the Editor
IEEE Software, March/April 2002
Quality For Real Engineers
For decades, experts have struggled to define quality. Edwards Deming said
that the only definition of quality that mattered was the consumer's. Joseph
Juran said that quality was fitness for use . Philip Crosby provided the
strictest definition of quality as "conformance to requirements."
Conformance to Requirements
Although they differ on the details, quality experts agree that the customer's
view of requirements is critically important. For that reason, I've found
Crosby's definition of "conformance to requirements" to be the most useful
definition in examining software quality. Taking into account many software
projects' tendency to elicit some but not all of the customer's complete
requirements, "requirements" cannot be interpreted solely as the written
requirements. Requirements must also include implicit requirementsthose
that the customer assumes regardless of whether the development team happens to
write them down. Thus, the working definition of quality that I use is
"conformance to requirements, both stated and implied."
The "ities" of Software Quality
In addition to specific functional requirements, software quality is also
affected by common nonfunctional characteristics that are often referred to as
the "ities." The ities that affect software's internal quality (quality visible
to the software's developers) include maintainability, flexibility, portability,
reusability, readability, scalability, testability, and understandability. The
ities that affect the software's external quality (visible to the customer)
include usability, reliability, adaptability, and integrity, as well as
correctness, accuracy, efficiency, and robustness.
Some of these characteristics overlap, but all have different shades of meaning
that are desired more in some cases and less in others. The attempt to maximize
certain characteristics invariably conflicts with the attempt to maximize
others. Figure 1 presents a summary of the ways in which external quality
characteristics affect each other.
Figure 1. Interactions between product quality external
These characteristics will be prioritized differently on different projects,
which means the software quality target is always changing. Finding an
optimal solution from a set of competing, changing objectives is
challenging, but that's part of what makes software development a true
From Product Quality to Project Quality
When software people refer to quality, we usually refer to the quality of the
software product we are producing. From a management perspective,
however, customers also have requirements for projects. I think it's
reasonable to draw an analogy from products to projects, conceiving project
quality as conformance to requirements, both stated and implied. Customers'
functional requirements for projects draw from a small number of possible
attributes, namely schedule, resources, cost, and quality of the product
produced. In some cases, a customer might prioritize cost higherin
others, schedule or product quality.
Additionally, project quality includes nonfunctional requirements such as
Efficiency: Minimal use of schedule, budget, and staff to deliver a
particular software product.
Flexibility: The extent to which the project can be modified to
deliver software other than that for which the project was originally
intended or to respond to changes in project goals.
Improvability: The degree to which project experiences can be fed
back into the project to improve project performance.
Predictability: The degree to which a project's cost, schedule, and
product quality outcomes can be forecast in advance.
Repeatability: The degree to which the project after the current
project can be conducted using practices similar to those used on the
Robustness: The degree to which the project will continue to function
in the presence of stressful environmental conditions.
Sustainability:The duration for which a project can continue using
its current practices.
Visibility:The ability of a customer to accurately determine project
status and progress.
These project characteristics interplay with each other just as the software
quality attributes do. Figure 2 shows the interactions. In addition to the
interactions shown in Figure 2, some of these project quality characteristics
tend to support or undermine the various product characteristics summarized in
Figure 2. Interactions between project quality
Different projects have different priorities among efficiency, flexibility,
improvability, and the other characteristics shown in
Figure 2. An established business might place high values on
efficiency, predictability, improvability, and repeatability. A start-up
company might place a higher value on robustness and visibility; it might
not value sustainability and repeatability at all. This suggests that there
isn't one best definition of project quality for all projects; the best
definition depends on the project's consumers and those consumers' specific
One difference between a craftsman and an engineer is that a craftsman defines
quality on his own terms, whereas an engineer defines quality through his
customers' eyes. The craftsman settles into a way of working that suits him
personally, while the engineer adapts his approach on each project to best
satisfy his customer's requirements.
Software engineering purists argue that software should always be produced to
the highest level of quality, by which they mean the highest levels of product
quality. End-user requirements certainly should be considered, but the
organization that builds and sells the software is another consumer whose
requirements must be taken into account. The product characteristics that
constitute quality to the end user do not necessarily satisfy the
software-developing organization's project quality requirements.
As Deming pointed out in Out of the Crisis, different consumers can
have different definitions of quality for the same product, and this applies as
much to project quality as it does to product quality. The project team,
manager, and sponsoring organization can all be considered consumers of a
project. A manager might consider a project to have high quality if it provides
good visibility, robustness, and repeatability. The project team might value
efficiency, improvability, and sustainability. The sponsoring organization might
value predictability and flexibility.
A manager who factors product quality into the project plans but ignores project
goals takes an abridged view of software quality. One hallmark of engineering
work is the constant balancing of trade-offs. With the extensive trade-off
decisions required to balance both software product attributes and software
project goals, software personnel have abundant opportunities to hone their
engineering skills in this area.
1. W. Edwards Deming, Out of the Crisis, MIT Press, 2000.
2. J.M. Juran, Juran's Quality Handbook, McGraw-Hill, 1998.
3. P.B. Crosby, Quality Is Free: The Art of Making Quality Certain,
Mentor Books, 1992.
4. S. McConnell, Code Complete, Microsoft Press, 1993.