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Estimating work is a key element of project planning. The following techniques can be used at the project level or activity level, or for any sized work in-between. For instance, an expert opinion can be used to help guide the estimate for an entire project or a specific piece of work.
High-level estimating techniques are typically referred to as top-down. Top-down techniques include prior history, analogy and ratio. Estimates that rely on a more thorough breakdown of the work are called bottom-up. The WBS technique, for example, is a bottom-up technique.
Top-down estimates are typically quicker and easier to put together, since you are estimating at the overall project level. They can also be less precise.
Previous History. If your organization keeps track of actual effort hours and costs from previous projects, you may have information that will help you estimate similar new work. In this method, the characteristics of the prior work, along with the actual effort hours and cost, are saved so that the information can be leveraged for future projects.
Analogy. Even if you do not keep actual effort hours from previous projects, you may still be able to leverage previous work. Analogy means that you find similar projects, even if the project team did not collect actual effort hours worked. For example, let's say a prior project was estimated to take six months and 2000 hours to complete. If the project actually was completed in six months, there is a good likelihood that the project also took approximately 2000 hours of effort.
Ratio. Ratio is similar to analogy except that you have some basis for comparing work that has similar characteristics, but on a larger or smaller scale. For instance, you may find that the effort required to complete an office move for the Miami office was 500 hours and that one of the big drivers for the effort is the number of people at the office. If there are twice as many people in the Chicago office, you may be able to conclude the work may take 1000 hours there.
Expert Opinion. In many cases you may need to go to an internal or external expert to get help estimating the work. For instance, if this is the first time you have used a new technology, you may need the help of an outside research firm to provide estimating information. Many times these estimates are based on what other companies in the industry are experiencing. You may also have an internal expert who can help. Although this may be your first time estimating a certain piece of work, perhaps someone else has done it many times.
Delphi. The Delphi technique is similar to expert opinion, except that you use multiple experts and try to reach an estimating consensus among them.
If possible, you should utilize multiple estimating techniques for a project, especially if you are using a quick top-down technique. You will have more confidence in your estimate if you use two or more techniques to arrive at similar estimates.
(Look for the remaining five estimating techniques next week.)