19.5 ISBSG Method

The International Software Benchmarking Standards Group (ISBSG) has developed an interesting and useful method of computing effort based on three factors: the size of a project in function points, the kind of development environment, and the maximum team size (ISBSG 2005). Presented by project type, the following eight equations are the ones you'd use to estimate effort using this approach. The equations produce an estimate in staff months, assuming 132 project-focused hours per staff month (that is, excluding vacations, holidays, training days, company meetings, and so on). The General formula is a general-purpose formula for use on all project types and is based on calibration data from about 600 projects. The other categories are calibrated with data from 63 to 363 projects.

Kind of Project: General

(#9)

Kind of Project: Mainframe

(#10)

Kind of Project: Mid-Range

(#11)

Kind of Project: Desktop

(#12)

Kind of Project: Third Generation Language

(#13)

Kind of Project: Fourth Generation Language

(#14)

Kind of Project: Enhancement

(#15)

Kind of Project: New Development

(#16)

Suppose you were creating an effort estimate for a desktop business application of 1,450 function points in Java (the same system we've been estimating throughout this chapter) and you have a maximum team size of 7 people. The Desktop equation suggests you will have an effort of 56 staff months:

0.157 × 1,450^0.591 × 7^0.810

You could also use the Third Generation Language equation to get an estimate of 58 staff months:

0.425 × 1,450^0.488 × 7^0.697

An interesting aspect of the ISBSG method is that the formulas for effort depend on the maximum size of the project team, with smaller teams producing smaller total effort estimates. Varying the maximum team size in the example from 5 to 10 people causes the effort estimate to vary from 43 to 75 staff months. From an estimation point of view, this introduces uncertainty. From a project control point of view, this difference might lead you to use a smaller team size rather than a larger one. (This topic is addressed further in "Schedule Compression and Team Size" in Section 20.6, "Tradeoffs Between Schedule and Effort.")

Tip #87

Use the ISBSG method to compute a rough effort estimate. Combine it with other methods, and look for convergence or spread among the different estimates.