Estimating activity duration accurately is important to the success of any project. You and your team estimate the various requirements of cost, time, and resources throughout your project. Although activity duration estimation looks at the time it takes to complete the entire project, activity duration estimation is dependent on other time and resource estimates.
You might start on these estimates at the inception of the project, but you make the most of the estimates during the planning phase. It's a good idea that all the people who implement the plan participate in preparing it.
Estimates are never exact. That's their nature — they are best guesses. But you can improve your accuracy by dividing the estimation task into three distinct steps: determining a Work Breakdown Structure (WBS), estimating the amount of work and duration of work packages, and calculating the project schedule.
Determine the Work Breakdown Structure (WBS)
The Work Breakdown Structure (WBS) is an estimating tool that defines a project in terms of its deliverables. The WBS also provides a method for breaking down deliverables into meaningful units of work. With this breakdown, you establish the work hierarchy and create a foundation for the other elements of the estimating process.
A WBS is divided into work levels. Work packages are at the lowest levels (or "end nodes") of each branch of your WBS. A work package usually refers to a unit of work to be performed within your organization.
Get your stakeholders involved when you start on your WBS. They can provide information on tasks and resources that might otherwise be overlooked. Generally, the more complete your WBS, the more accurate your duration estimate.
When creating your WBS:
- Develop your WBS before scheduling or allocating resources.
- Do not use your WBS to show the work sequence.
Note This happens later, when you create a network diagram or a schedule.
- Involve knowledgeable individuals in your WBS development.
- Break down your project only as far as you must for accurate estimation.
- Do not force all paths on your WBS down to the same level.
- Use no more than 20 levels.
Estimate the duration of work packages
The duration of a work package is equal to the amount of time required to do the work, divided by the number of people working (duration=work/number of people).
After you have the raw estimate for a work package, you need to add some padding to get a realistic number. The padding accounts for real-life factors such as:
- Lost time Team members and workers are not available five days per week, 52 weeks per year. Personal or public holidays, sickness, training, group meetings, family emergencies, and other events affect the time that workers can spend on the job. Keep these factors in mind when you calculate the duration of work packages. You can use enterprise calendars in project-tracking systems, such as Microsoft Office Project Professional 2003, to help you make accurate estimates that allow for these time losses.
- Part–time work Assigning part–time work on a given work package lengthens its duration. Half–time work doubles the duration, three–quarter work adds 50% to the duration, and so on. Consult personal schedules and work hours when calculating the effect of part–time work on your estimate.
- Interference Physical constraints or features of the work location can have a negative impact on effectiveness and increase work package duration. Areas with limited access or workspace might cause workers to get in each other's way or limit their use of facilities. For instance, in a room that accommodates only one worker at a time, adding another worker does not decrease duration, but might add to it instead. Fax machines, phones, noise, proximity of machinery, placement of equipment, and even traffic flow might cause interference. As interference rises, productivity falls and work duration rises. Take into account the possible interference factor when you make your duration estimate.
- Communication Communication takes time. While communication is good, trying to communicate too much between too many team members can increase work duration. The number of communication channels grows exponentially as the number of team members increases. Messages can also lag when distance and time–zone differences are involved. Include the communication factor in your estimate.
Experience and the use of historical data also improves your estimates. If you've done a task frequently — or have a lot of documentation for similar tasks — you'll know the task's average duration. After you know the level of human resources available to do the work, you can use this average in calculating your estimate. But because estimates are not exact, an average duration is only that — an average. In practice, the duration has a 50% chance of being higher or lower than the average.
With this in mind, it's tempting to pad your estimate a lot and increase the probability that the work is completed on time or ahead of time. But beware: Pad with care. Overpadding increases budget costs. If your budget is too fat, your project might not get funded.
Some projects don't lend themselves well to standard estimating techniques. Historical data becomes particularly useful in these cases. For instance, most software projects are not mechanistic and activities can be indeterminate, so estimating project duration for software development has always been difficult. But by tailoring their model to their needs and using historical data about similar projects, many software companies produce better estimates than ever before. Using historical data provides a key to better estimation.
Calculate the project schedule
When your WBS is complete and you have duration estimates for all the work packages, you and your project team can determine the overall project duration. The two most common estimating methods for this are the Critical Path Method (CPM) and the Performance Evaluation Review Technique (PERT).
The differences between CPM and PERT are:
- CPM calculates the total project duration based on individual task durations and their interdependencies. The sequence of tasks determining the minimum time needed for the project is the critical path.
- PERT is a pictorial description of project tasks as a network of dependencies. Although it is also concerned with critical paths, PERT looks at the most likely time estimates for tasks and boundary times (time windows) for tasks.
You can choose from many other methods, however, if neither CPM nor PERT meets your needs. Other estimating methods include:
- Delphi Method
- Expert judgment
- Function point counting
- Historical comparison
- Object based
- Scheduling heuristics
- Weighted Average (WAVE)
After you choose your method, you might need to make a network diagram. Both CPM and PERT rely on creating network diagrams that show the interdependencies of work packages according to a physical scheme.
The two leading schemes are:
- Activity-on-node networks (precedence networks) represent work packages as boxes, linked by logical dependencies, shown as arrows. These diagrams show how one element follows another.
- Activity-on-arrow networks represent work package as arrows between two nodes. These diagrams show the interdependency of elements.
Next, you need to know the early start and finish dates, the late start and finish dates, and the float. Calculate the early start and finish dates by conducting a forward pass through the network you've just made. Calculate the late start and finish dates and the float by doing a back pass.
When you've identified the early start and finish dates, the late start and finish dates, and the float, you and your project team can identify the critical path. The critical path is the shortest path on the project. If a task on the critical path is delayed, the entire project is delayed.
With all this information assembled, you can calculate your schedule and complete your estimate of project duration. Project management software can help you by creating diagrams, calculating schedules, and assisting you with your final estimate, while keeping track of complex interdependencies and multitudes of work packages.
A structured and disciplined approach to estimating project duration helps you create a comprehensive estimate that reflects real tasks and real-world scheduling. This approach removes uncertainty and creates a project plan that helps you know your options and adapt to path changes. Sound duration estimation also supports realistic budget estimates that increase your project's chance of success. Even though estimates are never exact, solid data and good estimating practices make your "best guesses" better.
About the author Pcubed is a global company that provides program management and project management solutions, as well as services in consulting, outsourcing, technology, and training.