Bottom-Up Estimating

Most real-world estimating is made using bottom-up estimating (Royce 1998). Bottom-up estimating involves dividing the project into smaller modules and then directly estimating the time and effort in terms of person-hours, person-weeks, or person-months for each module. The work breakdown structure provides the basis for bottom-up estimating because all of the project phases and activities are defined.

The project manager, or better yet the project team, can provide reasonable time estimates for each activity. In short, bottom-up estimating starts with a list of all required tasks or activities and then an estimate for the amount of effort is made. The total time and associated cost for each activity provides the basis for the project’s target schedule and budget. Although bottom-up estimating is straightforward, confusing effort with progress can be problematic (Brooks 1995).

Continuing with our earlier example, let’s assume that after meeting with our software testers, the following durations were estimated for each of the following activities:

6.2
Test results report
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 6.2.1. Review test plan with client
1 day
 6.2.2. Carry out test plan
5 days
 6.2.3. Analyze results
2 days
 6.2.4. Prepare test results report and presentation
3 days
 6.2.5. Present test results to client
1 day
 6.2.6. Address any software issues or problems
5 days

If we add all of the estimated durations together, we find that creating the test results report will take 17 days. How did we come up with these estimates? Did we guesstimate them? Hopefully not! These estimates could be based on experience—the software testers may have done these activities many times in the past so they know what activities have to be done and how long each activity will take. Or, these estimates could be based on similar or analogous projects. Analogous estimation refers to developing estimates based upon one’s opinion that there is a significant similarity between the current project and others (Rad 2002).

Keep in mind that estimates are a function of the activity itself, the resources, and the support provided. More specifically, the estimated duration of an activity will first depend upon the nature of the activity in terms of its complexity and degree of structure. In general, highly complex and unstructured activities will take longer to complete than simple, well structured activities.

The resources assigned to a particular activity will also influence an estimate. For example, assigning an experienced and well trained individual to a particular task should mean less time is required to complete it than if a novice were assigned. However, experience and expertise are only part of the equation. We also have to consider such things as a person’s level of motivation and enthusiasm.

Finally, the support we provide also influences our estimates. Support may include technology, tools, training, and the physical work environment.

These are just some of the variables that we must consider when estimating. You can probably come up with a number of others. Subsequently, estimates will always be a forecast; however, by looking at and understanding the big picture, we can increase our confidence in them.

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