20.3 Jones s First-Order Estimation Practice

20.3 Jones's First-Order Estimation Practice

If you have a function-point count, you can compute a rough schedule directly from function points by using what Capers Jones has described as the First-Order Estimation Practice (Jones 1996). To use it, take your function-point total and raise it to the appropriate power selected from Table 20-3. The Average exponents in the table are derived from Jones's analysis of several thousand projects. I've added approximate Better and Worse exponents to represent variations in performance.

Table 20-3: Exponents for Computing Schedules from Function Points

Kind of software	Better	Average	Worse
Object-oriented software	0.33	0.36	0.39
Client-server software	0.34	0.37	0.40
Business systems, internal intranet systems	0.36	0.39	0.42
Shrink-wrapped, scientific systems, engineering systems, public internet systems	0.37	0.40	0.43
Embedded systems, telecommunications, device drivers, systems software	0.38	0.41	0.44
Source: Loosely adapted from "Determining Software Schedules" (Jones 1995c) and Estimating Software Costs (Jones 1996).

If you estimate your project's total number of function points to be 1,450, and you're working in a business-systems organization with average productivity, you will raise 1,450 to the 0.39 power (1,450^0.39), for a rough schedule of 17 calendar months. If you are working in a best-in-class business-systems organization, you will raise 1,450 to the 0.36 power, for a schedule of 14 months. If you're developing an object-oriented business system, the object-oriented software exponents will give you a range of 11 to 17 months. Thus, it appears that the real schedule lies somewhere in the range of 11 to 17 months.

This practice isn't a substitute for more careful schedule estimation, but it does provide a simple means of getting a rough schedule estimate that's better than guessing. It can also provide a quick reality check. If you want to develop a 1,450-functionpoint business system in 9 months, you should think again. The best-in-class schedule would be 11 to 14 months, and most organizations aren't best in class. Jones's First-Order Estimation Practice allows you to know early on if you need to adjust your feature set expectations, schedule expectations, or both.

Tip #91 

Use Jones's First-Order Estimation Practice to produce a low-accuracy (but very low-effort) schedule estimate early in a project.