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136 PART TWO MANAGING SOFTWARE PROJECTS

To simplify the estimation process and use a more common form for their esti-
mation model, Putnam and Myers [PUT92] suggest a set of equations derived from
the software equation. Minimum development time is defined as

t min = 8.14 (LOC/P) 0.43 in months for t min > 6 months (5-4a)
E = 180 Bt 3 in person-months for E ≥ 20 person-months (5-4b)

Note that t in Equation (5-4b) is represented in years.
Using Equations (5-4) with P = 12,000 (the recommended value for scientific soft-
ware) for the CAD software discussed earlier in this chapter,

t min = 8.14 (33200/12000) 0.43
t min = 12.6 calendar months
E = 180 0.28 (1.05) 3
E = 58 person-months

The results of the software equation correspond favorably with the estimates devel-
oped in Section 5.6. Like the COCOMO model noted in the preceding section, the soft-
ware equation has evolved over the past decade. Further discussion of an extended
version of this estimation approach can be found in [PUT97b].

5.8 THE MAKE/BUY DECISION

In many software application areas, it is often more cost effective to acquire than
develop computer software. Software engineering managers are faced with a
make/buy decision that can be further complicated by a number of acquisition
options: (1) software may be purchased (or licensed) off-the-shelf, (2) “full-
experience” or “partial-experience” software components (see Section 5.4.2) may
be acquired and then modified and integrated to meet specific needs, or (3) soft-
ware may be custom built by an outside contractor to meet the purchaser's
specifications.
The steps involved in the acquisition of software are defined by the criticality of
There are times when the software to be purchased and the end cost. In some cases (e.g., low-cost PC soft-
off-the-shelf software ware), it is less expensive to purchase and experiment than to conduct a lengthy eval-
provides a “perfect” uation of potential software packages. For more expensive software products, the
solution except for a following guidelines can be applied:
few special features
that you can’t live 1. Develop specifications for function and performance of the desired soft-
without. In many ware. Define measurable characteristics whenever possible.
cases, it’s worth living
without the special 2. Estimate the internal cost to develop and the delivery date.
features!
3a. Select three or four candidate applications that best meet your specifications.
3b. Select reusable software components that will assist in constructing the
required application.