4.1.2 Bottom-Up Estimation Approach

A somewhat different approach for effort estimation is the bottom-up approach. In this approach, the project is first divided into tasks and then estimates for the different tasks of the project are obtained. From the estimates of the different tasks, the overall estimate is determined. That is, the overall estimate of the project is derived from the estimates of its parts. This type of approach is also called activity-based estimation. Essentially, in this approach the size and complexity of the project is captured in the set of tasks the project has to perform.
 

The bottom-up approach lends itself to direct estimation of effort; once the project is partitioned into smaller tasks, it is possible to directly estimate the effort required for them, especially if tasks are relatively small. One difficulty in this approach is that to get the overall estimate, all the tasks have to be enumerated. A risk of bottom-up methods is that one may omit some activities. Also, directly estimating the effort for some overhead tasks, such as project management, that span the project can be difficult.

If architecture of the system to be built has been developed and if past information about how effort is distributed over different phases is known, then the bottom-up approach need not completely list all the tasks, and a less tedious approach is possible. Here we describe one such approach used in a commercial organization [58].
 

In this approach, the major programs (or units or modules) in the software being built are first determined. Each program unit is then classified as simple, medium, or complex based on certain criteria. For each classification unit, an average effort for coding (and unit testing) is decided. This average coding effort can be based on past data from a similar project, from some guidelines, or on experience of people.
 

Once the number of units in the three categories of complexity is known and the estimated coding effort for each program is selected, the total coding effort for the project is known. From the total coding effort, the effort required for the other phases and activities in the project is determined as a percentage of coding effort. For this, from information about the past performance of the process, the likely distribution of effort in different phases of this project is determined. This distribution is then used to determine the effort for other phases and activities from the effort estimate of coding. From these estimates, the total effort for the project is obtained.
 

This approach lends itself to a judicious mixture of experience and data. If suitable past data are not available (for example, if launching a new type of project), one can estimate the coding effort using experience once the nature of the different types of units is specified. With this estimate, we can obtain the estimate for other activities by working with some reasonable or standard effort distribution. This strategy can easily account for activities that are sometimes difficult to enumerate early but do consume effort by budgeting effort for an “other” or “miscellaneous” category. 

The procedure for estimation can be summarized as the following sequence of steps:

1. Identify modules in the system and classify them as simple, medium, or complex.
2. Determine the average coding effort for simple/medium/complex modules.
3. Get the total coding effort using the coding effort of different types of modules and the counts for them.
4. Using the effort distribution for similar projects, estimate the effort for other tasks and the total effort.
5. Refine the estimates based on project-specific factors.

This procedure uses a judicious mixture of past data (in the form of distribution of effort) and experience of the programmers. This approach is also simple and similar to how many of us plan any project. For this reason, for small projects, many people find this approach natural and comfortable.
 

Note that this method of classifying programs into a few categories and using an average coding effort for each category is used only for effort estimation. In detailed scheduling, when a project manager assigns each unit to a member of the team for coding and budgets time for the activity, characteristics of the unit are taken into account to give more or less time than the average.