Estimating Considerations
When you prepare estimates, be sure you know the language used in estimating (for example, if you are talking about elapsed time, ensure that the sponsor is not talking about work time). This can make a major difference in the resulting costs. Estimates vary, depending on whether the task is based on effort or duration.
Most tasks that have work to be done as a criterion for completion are considered effort-based.
The task has a total amount of effort that must be completed in order to finish the task. This might also be referred to as staff effort and is generally expressed in person hours. As a rule, the more resources assigned to the task, the shorter its duration.
Some tasks are duration-based. The duration is constant regardless of how many resources are assigned to the task. (For example, the duration required for a slab of concrete to dry is the same regardless of the number of people watching it happen).
Another key difference is elapsed time as opposed to working time. Elapsed time is the total number of days over which the task occurs. This is also called calendar time, and is usually expressed in calendar days, weeks, or months. Working time is the actual amount of time available for work. Working time takes into account the working hours or time available for project team members. It does not include time away from the job such as vacation or holidays. Working time is usually measured in working hours divided by days or weeks.
Availability is the time a staff person is available and willing to work. This is usually measured in work hours per day or working days.
Productivity is a relative measure of work in a time unit. Different skill levels have different productivity rates. You must determine which productivity should be used for the estimate. The safest approach is to use an average productivity of 80%.
Keep in mind that the time it takes to complete a task depends on both availability and productivity.
Estimating Guidelines
Estimating enables you to determine effort and duration for the elements in the WBS. Some guidelines for preparing estimates are: