Estimation Techniques

One Time estimation:

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When estimating using a one-time estimate, one estimate per activity is received. The estimate can be made based on expert judgment, by looking at historical information, or even by just guessing.

One-time estimate per activity has the following negative effects on the project:

- Can force people into padding their estimates

- Hides important information about risks and uncertainties from the project manager, which are needed to better plan and control the project

- Creates a baseline that no one believes in, thus losing buy-in to the project management process

- When a one-time estimate is not met, it can make the person who provided the estimate look untruthful and untrustworthy

- Has the estimators working against the project manager

One-time estimates should only be used for projects that do not require a detailed, highly probable schedule. If one-time estimates are used, it is critical that the project manager provide the estimator with as much information as possible, including the WBS, WBS dictionary and the activity list, or the estimate will likely be unreliable.

Analogous Estimation:

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Also called "Top Down" high-level estimating. It can be done for a project (The last five projects similar to this one each took five months, so this one should also.) or an activity (The last two times this activity was completed each took three days. Since we have no other information to go on, we will use three days as the estimate for this activity and review the estimate when more details become available.). However, the exam seems to take an analogous estimate to mean only the overall project estimate given to the project manager from management or the sponsor. Analogous estimating is a form of expert judgment.

Advs of Analogous estimation:

- Quick

- Activities need not be defined

- Less costly to create

- Gives project manager an idea of the level of management's expectations

- Overall prject costs will be capped

Dis-advs:

- Less accurate

- Estimates are prepared with limited information and understanding of project

- Requires considerable experience

- Extremely difficult for projects with uncertainty

- Does not take into account differences between projects

Bottom-Up Estimation:

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Detailed estimating is done for each activity (if available) or work package (if activities are not defined) and the estimates are then rolled up into an overall project estimate.

Advs of Bottom-Up estimation:

- More accurate

- Gains buy-in from team because they create the estimates

- Based on detailed analysis of project

- Provides a basis for monitoring, controlling, performance measurment and management

Dis-advs of Bottom-Up estimation:

- Takes time and expense

- Requires the project be defined and well understood

- Requires time to break down project into smaller pieces

- Tendency for team to pad estimates

Parametric Estimation:

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Used if you do not have detailed information on which to base time estimates. It uses a mathematical model to calculate projected times for an activity based on historical records from previous projects and other information. The result is an activity estimate based on such measures as time per line of code, time per linear meter or time per installation.

There are two ways to create parametric estimates:

- Regression analysis (scatter 100 - diagram) This diagram tracks two variables to see if they are related and creates a mathematical formula to use in future parametric estimating.

- Learning curve The 100th room painted will take less time than the first room because of improved efficiency.

Heuristics :

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A heuristic means a rule of thumb. An example of a heuristic is the 80/120 rule that, in quality, suggests that 80 percent of quality problems are caused by 20 percent of potential sources of problems. The results of parametric estimates can become heuristics.

Three-Point Estimation: Also called PERT estimate