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5.6 Diseconomies of Scale Revisited

The Cocomo II adjustment factors provide an interesting viewpoint into how diseconomies of scale operate . In Figure 5-9, 5 of the factors in the figure are called scaling factors . These are the factors that contribute to software's diseconomies of scale. They affect projects to different degrees at different sizes. Figure 5-9 shows the same graph with these factors highlighted in blue.

image from book
Figure 5-9: Cocomo II factors with diseconomy of scale factors highlighted in blue. Project size is 100,000 LOC.

None of the factors that contribute to software's diseconomy of scale is in the top half of factors in terms of significance. In fact, 4 of the 5 least-influential factors are scaling factors. However, because the scaling factors contribute different amounts at different project sizes, this diagram must be drawn from the point of view of a project of a specific size. The factors in Figure 5-9 are shown for a project of 100,000 lines of code. Figure 5-10 shows what happens when the factors are recalculated for a much larger project of 5,000,000 lines of code.

image from book
Figure 5-10: Cocomo II factors with diseconomy of scale factors highlighted in blue. Project size is 5,000,000 LOC.

The scaling factors all become significant as project size increases . Although none of them was in the top half at 100,000 LOC, all the scaling factors are in the top half at 5,000,000 LOC.

What this means from an estimating point of view is that different factors need to be weighted differently at different project sizes. What this means from a project planning and control point of view is that small and medium- sized projects can succeed largely on the basis of strong individuals. Large projects still need strong individuals, but how well the project is managed ( especially in terms of risk management), how mature the organization is, and how well the individuals coalesce into a team become as significant.

Additional Resources

Boehm, Barry, et al. Software Cost Estimation with Cocomo II . Reading, MA: Addison-Wesley, 2000 . This book is the definitive description of Cocomo II. The book's size is daunting, but it describes the basic Cocomo model within the first 80 pages, including detailed definitions of the effort multipliers and scaling factors discussed in this chapter and how Cocomo II accounts for diseconomies of scale. The rest of the book describes extensions of the model .

Putnam, Lawrence H. and Ware Myers. Measures for Excellence: Reliable Software On Time, Within Budget . Englewood Cliffs, NJ: Yourdon Press, 1992 . This book describes Putnam's estimation method including how it addresses diseconomies of scale. I like Putnam's model because it contains few control knobs and works best when it is calibrated with historical data. The book is mathematically oriented, so it can be slow going.