Section 24. Costs

24. Costs

The other cost of requirements is the amount of money or effort that you have to spend turning them into a product. Once the requirements specification is complete, use one of the estimating methods to assess the cost, and then express this cost as a monetary amount, or effort or time to build the product.

There is no best method to use when estimating. The important thing is to create your estimates using metrics directly related to the requirements. If you have specified the requirements in the way we have described, you will have the following metrics:

• Number of business events

• Number of product use cases

• Number of requirements

• Number of constraints

• Number of function points

You can use these metrics as the basis for estimating the time, effort, and cost of building the product. First you need to determine what each of these metrics means within the environment in which you are building the product. For example, do you know how long it will take you to do all the work necessary to implement a product use case? If you do not, then you can take one of the use cases and benchmark it.

Garmus, David, and David Herron. Function Point Analysis: Measurement Practices for Successful Software Projects. Addison-Wesley, 2001.

appendix C, Function Point Counting: A Simplified Introduction, describes function point counting as it can be used by requirements analysts.

We favor counting function points as a means to estimate the size of the problem. This approach is a commonly accepted sizing method, and so much is known about it that it is possible to make easy comparisons with other products and with other installations' productivity.

At this stage, your client needs to know what the product is likely to cost. You usually express this as a total cost to complete the product, but you may also find it advantageous to point out the costs of individual requirements.

Whatever you do, do not leave the costs in the lap of hysterical optimism. Make sure that this section includes realistic numbers and coherent estimates and questions based on those numbers.