SOFTWARE DESIGN FOR X - Software Design for Six-Sigma: A Roadmap for Excellence - page 386

The bath tub curve is generated by mapping the rate of early “infant mortality”
failures when first introduced, the rate of random failures with a constant failure
rate during its “useful life,” and finally the rate of “wear out” failures as the product
exceeds its design lifetime.
In less technical terms, in the early life of a product adhering to the bath tub curve,
the failure rate is high but rapidly decreasing as defective products are identified
and discarded, and early sources of potential failure such as handling and installa-
tion error are surmounted. In the mid-life of a product—generally, once it reaches
consumers—the failure rate is low and constant. In the late life of the product, the
failure rate increases as age and wear take their toll on the product. Many consumer
products strongly reflect the bath tub curve, such as computer processors.
For hardware, the bath tub curve often is modeled by a piecewise set of three
hazard functions:

⎧
⎨ c o − c 1 t + λ 0 ≤ t ≤ c o / c 1

h ( t ) = λ c o / c 1 < t ≤ t o (14.A.4)

⎩

c 2 ( t − t o ) + λ t o < t

For software, you can replace the piecewise approximation by the applicable
hazard function from Table 14.2: Software Reliability Growth Models and in light of
Figure 14.1.

REFERENCES

Biddle, Robert and Temperd, Ewan (1998), “Evaluating Design by Reusability,” Victo-
ria University of Wellington, New Zealand. http://www.mcs.vuw.ac.nz/research/design1/
1998/submissions/biddle/.
DeMillo, R.A., Lipton, R.J., and Sayward, F.G. (1978), “Hints on test data selection: Help for
the practicing programmer.” Computer , Volume 11 , #4, pp. 34-41.
Design for Test (2009), Wikipedia, the Free Encyclopedia . http://en.wikipedia.org/wiki/
Design for Test.
Dugan J.B. and Lyu, M.R. (1995), “ Dependability Modeling for Fault-Tolerant Software and
Systems ,” in Software Fault Tolerance, Wiley & Sons, pp. 109-137.
ANSI/IEEE Std. 610.12-1990 (1990), “Standard Glossary of Software Engineering Terminol-
ogy,” IEEE, Washington, DC.
Kapur, K.C. and Lamberson, L.R. (1977), “ Reliability In Engineering Design ,” John Wiley &
Sons, Inc., New York
Keene, S. and Cole, G.F. (1994), Reliability growth of fielded software, Reliability Review ,
Volume 14 , pp. 5-26.
Keiller, P. and Miller, D. (1991), “On the use and the performance of software reliability
growth models.” Software Reliability and Safety , pp. 95-117.
Morris, Chang and Agun, Kagan (2000), “On Design-for-Reusability in Hardware Description
Languages,” VLSI Proceedings, IEEE Computer Society Workshop, Apr.