SOFTWARE DESIGN FOR X - Software Design for Six-Sigma: A Roadmap for Excellence - page 367

TABLE 14.2 ( Continued )

Formula for
Hazard Function

Data and/or Estimation
Required

Limitations and
Constraints Model Name

Either time between
failure occurrences X i
or the time of the
failure occurrence
Estimation of constant
, which decreases in
geometric progression
(0 <φ< 1) as failures
are detected.

Software is
operational.
Inherent number of
faults assumed to be
infinite.
Faults are independent
and unequal in
probability of
occurrence and
severity.

φ ι − 1

Geometric model

Number of failures
detected in each
interval (f i )
Length of testing time
for each interval i (T i )

Software is corrected
at end of testing
interval.
Software is
operational.
Software is relatively
fault free.

Thompson and
Chelson's
Bayesian Model

(f i + f 0 + 1)/ (T i +

T 0 )

most reliability growth equations assume that as time increases, reliability increases,
and the failure intensity of the software decreases. Instead of having a reliability theory
that makes these assumptions, it would be better to have a reliability measure that
actually had these considerations built into it. The notion of time is only peripherally
related to testing quality. Software reliability models typically ignore application
complexity and test coverage.
Software failures may be a result of errors, ambiguities, oversights, misinterpre-
tations of the specification that the software is supposed to satisfy, carelessness or
incompetence in writing code, inadequate testing, incorrect or unexpected usage of
the software, or other unforeseen problems (Keiller & Miller, 1991). Reliable software
has the following three characteristics:

A. Operates within the reliability specification that satisfies customer expecta-
tions. This is measured in terms of failure rate and availability level. The goal
is rarely “defect free” or “ultrahigh reliability.”
B. “Gracefully” handles erroneous inputs from users, other systems, and transient
hardware faults and attempts to prevent state or output data corruption from
“erroneous” inputs.
C. Quickly detects, reports, and recovers from software and transient hardware
faults. Software provides system behavior as continuously monitoring, self-
diagnosing,” and “self-healing.” It prevents as many run-time faults as possible
from becoming system-level failures.