Estimating Activity Durations - Project Management

Estimating activity durations is the next activity we'll undertake after constructing the WBS and RAM. The reasons why you constructed the WBS and the RAM first are so that you know which tasks you need estimates for and you know which resources you can ask to help you determine those estimates.

Activity duration estimates determine the number of work periods needed to complete the tasks defined in the WBS. Work periods are expressed in hours, days, weeks, or months. Hours and days are the most common work periods used, but you may need to use weeks or months if your project is large or is expected to take a long time to complete.

There are several techniques for determining activity duration estimates. A few of them are interchangeable with the budget estimating techniques we'll discuss in later.

Expert Judgment

Expert judgment is just what it sounds like. When you've determined the type of resource needed to perform the task, you can ask staff members who are experienced at these types of activities to give you an estimate for those tasks. Because of their experience with similar activities in the past, they'll be able to give you a fairly decent estimate. However, this is not a scientific method, and the person giving you the estimate may historically over-or underestimate durations based on their biases. To help even out these biases, you could ask more than one expert for an estimate and then combine their results. If possible, combine their expert judgment with historical information from past projects (remember those project notebooks filed away with all that juicy project information waiting for you to use as a reference?) to determine an estimate.

expert judgment

Using individuals or groups of people who have training, specialized knowledge, or skills to help assess information and determine estimates.

Quantitatively Based Durations

This type of estimate works with known quantities and calculates estimates based on the quantity of elements needed to complete the task. For example, you know that it takes six minutes to seal, stamp, and print the address on 5,000 brochures. If you're going to mail 50,000 brochures, you calculate the duration for this task by multiplying six minutes times ten to come up with a total duration of sixty minutes to seal, stamp, and print 50,000 brochures.

These duration estimates are initial estimates right now. You will have a chance to refine these, and you should refine these again when you create the project schedule. At that time, you'll have more information about the tasks, and your project team will be in place, so you'll have what you need to fine-tune these estimates.