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Optimum Quality Costs and
Zero Defects: Are They
Contradictory Concepts?
by
Arthur M. Schneiderman
A program of
continuous improvement
does not necessarily
introduce increased costs
as the quality level
approaches 100%
©
1986 American Society for Quality Control
S DEFECT LEVELS DROP
, failure costs decline while
appraisal plus prevention costs increase. This
apparent tradeoff suggests that an optimum quality level
exists and that attempts to further improve quality above this
level will increase total cost and decrease financial
performance. Proponents of this view therefore argue that
striving for zero defects (ZD) through a program of
continuous improvement is not in a company's best
economic interest.
J.M. Juran discusses the concept of optimum quality in his
Quality Control Handbook
.1, 2 Figure 1 depicts his model
for optimum quality costs. Juran also defines three quality
zones relative to the point of minimum total quality costs.
The “zone of improvement projects” lies below the optimum
quality level, while the “zone of perfectionism” lies above it.
Between them, and in the area of the minimum, lies the
“zone of indifference.” It is the zone of perfectionism that
most troubles proponents of zero defects, for here Juran
suggests relaxing prevention efforts and allowing (even
encouraging) increased defect rates. Furthermore, he
identifies the boundary of the zone of perfectionism as lying,
typically, at a quality level where failure costs amount to
40% of the total quality cost. Applying other rules of thumb,
this translates into a defect level only half that which exists
in the zone of improvement.
A
C2
Quality Progress/November 1986
Coming from anyone other than Juran, this apparent
heresy might go unnoticed. Are the ZD movement and the
concept of continuous improvement wrong? Or can these
two apparently disparate views be reconciled? To answer
this question, consider the “physics” involved in quality
cost optimization. First, the mathematics of optimization:
Let: f(q) = Total (internal + external) ~
failure costs
p(q) = Total (appraisal + prevention)
prevention costs A
T(q) = Total quality cost = f(q) + c
P(q)
q = quality level (0 to 100% good To
product)
Then, T(q) is minimized
3 when dT/dq = Q.
0 or dp/dq = -df/dq.
In other words, at the point of minimum total quality
costs, an additional dollar invested in prevention will
produce exactly one dollar's worth of reduced failure
costs. Below the optimum, this incremental dollar's worth
of prevention provides more than the proverbial dollar's
worth of cure. Above it, the opposite is true. There are
two other crucial lessons to be learned:
1. Optimum quality costs depend on incremental, not
total, elementary costs. At the optimum, nothing in
general can be said about the relative levels of prevention
and failure costs.
2. There is no mathematical requirement that the
optimum occurs at q < 100%. There may be no optimum
in the range of q = 0 to 100%. There might be a minimum
rather than an optimum, and it could very well be at q =
100%. Figure 2 shows an example.
The optimum (or more correctly, the minimum) quality
cost could lie at zero defects (q =100% ) if the
incremental cost of approaching ZD is less than the
incremental return from the resulting improvement. Juran
asserts that “prevention costs rise asymptotically,
becoming infinite at 100% conformance.”4 This implies
that the incremental cost is also infinite. Since the incremental
return is not, it follows from his assertion and
the above mathematics that the optimum lies below
100%. The question now is, “Does it really take infinite
investment to reach zero defects?”
ERO DEFECTS ADVOCATES
endorse continuous
improvement. This is the never-ending effort to
totally eliminate all forms of waste (the Japanese call it
“muda”), including reworks, yield losses, unproductive
time, over-design, inventory, idle facilities, safety
accidents, and the less tangible factors of unrealized
individual and societal potential. The methods used in this
process are widely misunderstood and in fact may lie
Z
Juran's model of
Optimum Quality Costs
Failure
Costs
Total
Quality
Costs
Costs of Appraisal
Plus Prevention
0 100 %
(100 % defective) (100 % good)
Cost Per Good Unit of Product
From J.M. Juran’s Quality Control Handbook,
Third Edition (New York: McGraw Hill, 1979) p. 5-12.
1
2
Quality Level
Failure
Costs
Total
Quality
Costs
Costs
of Appraisal
Plus Prevention
Quality Level
0 100 %
(100 % defective) (100 % good)
Cost Per Good Unit of Product
Optimum Quality Level
Equals Zero Defects
Quality Progress
/November 1986
C3
3
Two Contrasting Improvement Processes
Kaizen vs. Innovation
FOCUS
Design, Production
and Marketing
Science and
Technology
TARGETING
Broad:
Quality, Cost
Safety, Efficiency
Product Development
Narrow:
Feature
Technique
EXPERTISE
Conventional
Know-How
Leading Edge,
Breakthrough
CAPITAL NEEDS
Very Modest Major
Investment
PROGRESS
Small Steps Big Jumps
RESULTS
Continuous Spontaneous
VISIBILITY
Not Dramatic Very Dramatic
INVOLVEMENT
Everyone Selected Few
COOPERATION
Group Activity Individual
Effort
RECOGNITION
Effort, Process Results
Evolution Revolution
at the root of the issue. The Japanese word for
continuous improvement is “kaizen.” Figure 3
compares this method to an alternative improvement
process-innovation.
5 While innovation is
characterized by costly major events, kaizen
represents inexpensive and almost imperceptible
continuous improvement.
Kaizen is much like the tortoise in the fable of the
tortoise and the hare. It often beats innovation in the
race for competitive advantage. One striking
example was the reduction of dip soldering failures
at Yokogowa Hewlett-Packard (YHP), shown in
Figure 4.6 For a little more than two years, the
continuous improvement process on average
produced a 50% reduction in the failure rate every
3.6 months. Defects were reduced by a factor of
over 250. The process eventually slowed, probably
due to equipment limitations. Interestingly, that equipment
had purportedly been discarded as obsolete by a sister plant
in the U.S. It would not be at all surprising to find that the
equipment had become obsolete because of an innovation
that resulted in improvements of a factor of only two or
three.
What was the incremental cost to YHP in going from a
defect rate of 3 ppm to 2 ppm? What was the incremental
return? A detailed cost analysis could probably capture all of
the costs and benefits, but the results can be guessed. The
incremental costs are essentially zero. Why? At a minimum,
one could argue, there are the labor costs associated with the
time spent working on the improvements. But these were not
incremental or increased costs. They were fixed costs based
on a process that encourages everyone to spend about 5 to
10% of their time working on improvements.
What if less time were spent? Evidence suggests that this
would result in backsliding (Figure 5).7 After 20 months of
C4
Quality Progress/November 1986
continuous improvement (at an improvement rate of 50% each
5.1 months) and a tenfold reduction in scrap, the problem was
declared solved and all efforts toward further improvement
abandoned. The result: the gains could not be held, and the
scrap rate increased until the continuous improvement program
was reinstated. Quality is not a stable property. Without
constant effort from everyone, the organization naturally drifts
toward poor performance: higher cost and lower quality.
If the incremental labor costs are indeed zero, what about
capital costs required for these improvements? Again, they are
probably negligible. Kaizen-type improvement is usually the
result of better methods or small equipment changes or
additions.
The direct incremental benefits of continued improvement
are clearly small in going from 3 to 2 ppm. However, there are
some major cultural advantages: organizational pride,
reputation, spillover into other areas, and experience in
problem solving, to mention a few.
The correct way to view quality cost optimization is on the
basis of incremental economics. However, as ZD is
approached, it becomes harder to quantify any increased costs
or benefits as less tangible issues enter the equation. A
program of continuous improvement does not necessarily
introduce increased costs as the quality level approaches
100%.
Any benefit at all could produce a minimum quality
cost at zero defects. The apparent contradiction therefore
disappears once the underlying economics of both concepts are
clear. Perhaps the best test of this view is the competitive
performance of firms that believe in continuous improvement
and zero defects. That group includes not only Toyota and
Sony, but also IBM, Hewlett-Packard and an ever-increasing
number of successful U.S. firms.
References
1. J.M. Juran,
Quality Control Handbook, Third Edition (New
York McGraw-Hill, 1979), p. 5-11.
2. J.M. Juran, “Optimum Revisited,” Quality Progress, April
1986, p. 10.
3. The minimum value of a function is determined by the
point at which its slope is zero, or using calculus, when its first
derivative vanishes.
4. J.M. Juran, Quality Control Handbook, p. 5-12.
5. This comparison was first made by Masaaki Imai,
president, The Cambridge Corporation, Tokyo, Japan.
6. Kenzo Sasaoka, “Our TQC Experience as a Partner of
America,” presented at the 1984 Seminar and Plant Tour to
Study Productivity of Japanese Industry, sponsored and
organized by The Cambridge Corporation, Tokyo, Japan.
7. J.M. Juran, Management of Quality, Fourth Edition (New
York J.M. Juran, 1982), p. Jb9.
About the Author
Arthur M. Schneiderman is
group director, quality and
productivity improvement, Components Group, Analog
Devices, Wilmington, Mass. He has BS and MS degrees in
mechanical engineering from Massachusetts Institute of
Technology, as well as an MS in management from MlT's
Sloan School.
0 12 24 36 48 60
Failure Rate %
1
.1
.01
.001
.0001
Months
10,000
1,000
100
10
1
PPM
50 % Improvement Each:
3.6 Months
0 12 24 36
100
10
1
0.1
Scrap %
Months
50 % Improvement Each:
5.1 Months
4
Example of
Continuous Improvement
5
Failing to Hold the Gains
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