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Activity Duration Estimation Basics for the PMP Certification Exam

For PMP Certification Exam and real life purposes, you will need to have a handle on estimating project duration. Developing accurate estimates — for resources, durations, or costs — is one of the most challenging and contentious parts of managing a project. The customer always wants the deliverables as soon as possible. Team members assume everything will go as planned. Project managers assume resources will be available as promised.

All the above contribute to projects running longer than they were originally scheduled to run. The sponsor and the customer usually drive the project manager to unrealistic expectations, things don’t go as planned, and resources aren’t always available. No wonder projects run late even though team members work overtime trying to get caught up! To get out of this vicious cycle, it helps to understand the nature of estimating:

  1. You must fully define project and product scope to have any hope of estimating accurately, whether those estimates are for resources, durations, or costs.

    Oftentimes, you will be held to an estimated delivery date when the scope isn’t even finalized.

  2. Understand the nature of estimating.

    The more you understand about the scope, available resources, the nature of the work, and the environment, the more accurate your estimates will be. Therefore, at the beginning of a project, you are likely to have a range of estimates that can end up being 35–50% more optimistic than in the final duration. This is even truer with projects that are developing new technology or using unproven processes.

    Often, you can’t derive an accurate estimate until you’re ready to obtain a baseline and take into account your risks, stakeholder management needs, funding availability, procurement needs, actual resources, and so forth. Then you can get an estimate that is +/–5% to 10% of the final result.

  3. Take into account the difference between the effort needed to accomplish the work and the duration, which indicates how many work periods (activity duration) it will take.

    For example, say you have 80 hours of effort and two resources who are working full-time. You can assume that it will take 5 days, or 1 business week, to accomplish the work. If you have only 1 resource working at 50%, it will take 20 days, or 1 business month, to get the work done.

    Calculations like this get even more complicated if you consider that not all resources work at the same rate. Some people are more productive, and some people require more rework to get things right. In addition, for every 40 hours spent at work per week, folks are productive only about 34 hours.

If you consider all these elements, it’s no wonder why projects come in late! Still, that’s not reason enough to give estimating less than your best shot. Several estimating techniques are available that can help you develop estimates, depending on the nature of the work.

Effort. The number of labor units required to complete a schedule activity or work breakdown structure component, often expressed in hours, days, or weeks.

Duration. The total number of work periods (not including holidays or other nonworking periods) required to complete a schedule activity or work breakdown structure component. Usually expressed as workdays or workweeks. Sometimes incorrectly equated with elapsed time.

Elapsed time refers to calendar time. Thus, a duration of 20 days would be the equivalent of 4 workweeks. Most scheduling software turns duration into elapsed time by assuming that your work calendar doesn’t include weekends. You can change this by adjusting the calendar on your scheduling software. Or, you can enter the duration with a suffix that indicated elapsed days. For example, 20ED could mean 20 elapsed days.

You need to know only effort and duration. You do not need to consider elapsed time for the exam.

Estimate Activity Durations. Estimating the number of work periods needed to complete individual activities with estimated resources.

To determine effort and duration, you need to incorporate information from the previous time-management processes:

  • Schedule management plan

  • Activity list

  • Activity attributes

  • Activity resource requirements

  • Resource breakdown structure

The scope statement includes assumptions and constraints that you should take into consideration, such as availability or resources, available skills, contractual information, and so forth.

Your resource calendars will provide availability and resource load information for both internal resources and external resources (for example, if you’re using contract labor or rented equipment).

The risk register can provide information on WBS elements that have risks associated with delivery dates, materials, technology, resource availability, and any other type of risk that could impact duration estimates.

Organizations that have a degree of maturity in project management might have EEFs such as estimating databases, productivity metrics, or published estimating data. Some of the OPAs you’re likely to use include historic information from past projects and “lessons learned” information.

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