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140 PART TWO MANAGING SOFTWARE PROJECTS

When different estimation tools are applied to the same project data, a relatively
large variation in estimated results is encountered. More important, predicted values
sometimes are significantly different than actual values. This reinforces the notion
that the output of estimation tools should be used as one "data point" from which
estimates are derived—not as the only source for an estimate.

5.10 SUMMARY
The software project planner must estimate three things before a project begins: how
long it will take, how much effort will be required, and how many people will be
involved. In addition, the planner must predict the resources (hardware and software)
that will be required and the risk involved.
The statement of scope helps the planner to develop estimates using one or more
techniques that fall into two broad categories: decomposition and empirical model-
ing. Decomposition techniques require a delineation of major software functions, fol-
lowed by estimates of either (1) the number of LOC, (2) selected values within the
information domain, (3) the number of person-months required to implement each
function, or (4) the number of person-months required for each software engineer-
ing activity. Empirical techniques use empirically derived expressions for effort and
time to predict these project quantities. Automated tools can be used to implement
a specific empirical model.
Accurate project estimates generally use at least two of the three techniques just
noted. By comparing and reconciling estimates derived using different techniques,
the planner is more likely to derive an accurate estimate. Software project estima-
tion can never be an exact science, but a combination of good historical data and
systematic techniques can improve estimation accuracy.

REFERENCES

[BEN92] Bennatan, E.M., Software Project Management: A Practitioner’s Approach,
McGraw-Hill, 1992.
[BOE81] Boehm, B., Software Engineering Economics, Prentice-Hall, 1981.
[BOE89] Boehm, B., Risk Management, IEEE Computer Society Press, 1989.
[BOE96] Boehm, B., “Anchoring the Software Process,” IEEE Software, vol. 13, no. 4,
July 1996, pp. 73–82.
[BOE00] Boehm, B., et al., Software Cost Estimation in COCOMO II, Prentice-Hall, 2000.
[BRO75] Brooks, F., The Mythical Man-Month, Addison-Wesley, 1975.
[GAU89] Gause, D.C. and G.M. Weinberg, Exploring Requirements: Quality Before
Design, Dorset House, 1989.
[HOO91] Hooper, J. and R.O. Chester, Software Reuse: Guidelines and Methods, Plenum
Press, 1991.