Information Technology Reference
In-Depth Information
Useful life
Time
he bathtub curve
Figure 1.1
The bathtub curve.
Until the 1970s, the bathtub curve remained the basis of scheduled
maintenance for physical machines and production plants, where parts of
such systems were regularly replaced regardless of their wear and tear.
All this changed in 1978 due to the work of Nowlan and Heap. They
worked with many years of data obtained from the airline industry and
disproved the concept of a
. Their studies
revealed (Figure 1.2) that greater than 70 to 90 percent of all failures were
random, leaving only a very small percentage of age-related failures. These
failure patterns made it clear that scheduled maintenance could have the
reverse effect on the life of a system. Any intrusion (part replacement)
could potentially cause more damage because the system would reset to
its infant mortality failure rate (which is higher) from its random failure
rate. Nowlan and Heap created the concept of Reliability Centered Main-
tenance, a concept later extended to other systems by Moubray in 1997.
His idea is that as more bugs and problems accumulate in a system, its
reliability starts deteriorating. As experienced managers and engineers, we
need to know when to schedule component replacements and perform
system maintenance; and Chapter 12 (on quality) further discusses this.
defined useful life in systems
The Dependability of a Software System
Dependability is a combination of reliability — the probability that a
system operates through a given operation specification — and availability
— the probability that the system will be available at any required instant.
Most software adopts the findings of Nowlan and Heap with respect to
failures. During the initial stages of the system, when it is being used
