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Analysis and Design
Measurement - Software Quality Assurance Activities
Earlier in this chapter we discussed what is the objective and meaning of the
quality with respect to software development. We defined a set of qualitative
factors of the software quality measurement. Since there is no such thing as absolute
knowledge we can’t expect to measure software quality exactly. Therefore
SQ metrics is developed and employed to figure out the measure, content of the
quality. A set of software metrics is applied to the quantitative assessment of
software quality. In all cases these metrics use indirect measures therefore quality
as such in itself is never measured but rather some manifestation of it.
There are a number of software quality indicators that are based on the measurable
design characteristics of a computer program. Design structural quality index
(DSQI) is one such measure. The following values must be ascertained to compute
the DSQI
S1 = the total number of modules defined in the program architecture
S2 = the number of modules whose correct function depends on the source of
data input or that produces data to be used elsewhere {in general control modules
(among others) would not be counted as part of S2}
S3 = the number of modules whose correct function depends on prior processing
S4 = the number of database items (includes data objects and all attributes
that define objects)
S5 = the total number of unique database items
S6 = the number of database segments ( different records or individual objects)
S7 = the number of modules with a single entry and exit ( exception processing
is not considered to be a multiple exit)
When all these values are determined for a computer program, the following
intermediate values can be computed:
Program structure: D1, where D1 is defined as follows:
If the architectural design was developed using a distinct method( e.g., data
flow-oriented design or object oriented design), then D1 = 1; otherwise D1 = 0.
Module independence: D2 = 1 -(S2/S1)
Module not dependent on prior processing: D3 = 1- (S3/S1)
Database size : D4 = 1- (S5/S4)
Database compartmentalization: D5 = 1- (S6/S4)
Module entrance/exit characteristic: D6 = 1- (S7/S1)
With the intermediate values determined, the DSQI is computed in the following
manner: DSQI = Swi Di
Where i = 1 to 6, wi is the relative weighting of the importance of each of
the intermediate values, and Swi = 1 ( if all Di are weighted equally, then wi
= 0.167).
The value of DSQI for past designs can be determined and compared to a design
that is currently under development. If the DSQI is significantly lower than average,
further design work and review is indicated. Similarly, if major changes are to
be made to an existing design, the effect of those changes on DSQI can be calculated.
IEEE Standard 982.1-1988 suggests a software maturity index (SMI) that provides
an indication of the stability of a software product ( based on changes that occur
for each release of the product). The following information is determined:
MT = the number of modules in the current release
Fc = the number of modules in the current release that have been changed
Fa = the number of modules in the current release that have been added
Fd = the number of modules from the preceding release that were deleted in
the current release
The software maturity index is computed as :
As SMI approaches 1.0, the product begins to stabilize
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