20.2 Computing Schedule by Using Informal Comparisons to Past Projects

William Roetzheim proposes estimating schedules of new projects based on a ratio of schedule and effort from past projects (Roetzheim 1988, Stutzke 2005). We'll use the same projects that were used in Chapter 19, "Special Issues in Estimating Effort," which are repeated in Table 20-1 for reference.

Table 20-1: Example of Past Project Efforts and Schedules for Estimating Future Schedules
Project	Size (LOC)	Schedule (Calendar Months)	Effort (Staff Months)	Productivity (LOC/Staff Month)	Comments
Project A	33,842	8.2	21	1,612
Project B	97,614	12.5	99	986
Project C	7,444	4.7	2	3,722	Not used—too small for comparison
Project D	54,322	11.3	40	1,358
Project E	340,343	24.0	533	639	Not used—too large for comparison

Roetzheim suggests estimating schedule in months by using the Informal Comparison to Past Projects formula:

(#18)

The exponent of 1/3 is used for what this book calls medium-to-large projects (more than about 50 staff months). For smaller projects, you should use an exponent of 1/2.

Chapter 19's effort estimate of 65 to 100 staff months, with a most likely estimate of 80 staff months,gives us the estimated schedules from past projects shown in Table 20-2.

Table 20-2: Example of Schedules Estimates Computed Using Informal Comparisons to Past Projects
Historical Data			Estimates
Project	Past Schedule (Calendar Months)	Past Effort (Staff Months)	Low Estimate (65 Staff Months)	Nominal Estimate (80 Staff Months)	High Estimate (100 Staff Months)
Project A	8.2	21	12.0	12.8	13.8
Project B	12.5	99	10.8	11.6	12.5
Project D	11.3	40	13.2	14.2	15.3

Roetzheim's technique produces a best-case to worst-case range of 10.8 to 17.3 months. I would compute the Expected Case by simply averaging the three nominal estimates from the table. I would compute the low and high ends of the range by averaging the low estimates and the high estimates from the table. Those calculations produce a nominal schedule of 12.9 months with a range of 12.0 to 13.9 months.

Tip #90

Use the Informal Comparison to Past Projects formula to estimate schedule early in a small-to-large project.