Top-Down Estimating
Top-down estimating results in high-level estimates of projects or their summary tasks based on parametric, analogy or comparison, or expert judgment. This approach is based on collecting judgments, past experiences, and on evaluating past data concerning similar activities. It is commonly used and is considered to be less costly though less accurate than other techniques. A top-down estimate is more reliable if the previous projects are similar in appearance and fact, and the individuals or groups preparing the estimate have the required expertise.
Top-down estimates are generally given to lower-level managers. These managers then break the estimate down for specific tasks and work elements that comprise subprojects, so the approach feeds other estimates.
Top-down estimating is considered a good approach to use during a proposal phase or when estimating a phase that is months or years away. If you lack detailed information or time, the top-down approach might be the preferred method. Documentation of the appropriate assumptions is critically important.
Remember that personal feelings can negatively influence a top-down estimate. Therefore, keep in mind that when you use this method, it is very easy to create an estimate that matches the results you have in mind.
Estimation Methods
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