Defining activities is an important part of scheduling. Part of the complexity of the project is determined by the number of activities required to complete that project. When it comes to the size and/or placement of the activities, several control factors should be considered. Once the project has been broken down into unique definable elements of work, the duration of these elements must be accurately estimated. Accurate definition of activities and their durations creates all the elements necessary for a believable schedule.
Activity Definition Schools of Thought
There are generally two schools of thought for defining the activities in a schedule. The first school of thought says to limit the number of activities in a project schedule to keep the schedule and corresponding Gantt chart simple. The length and duration of activities in this case are only restricted by contractual obligations that may stipulate the maximum duration of any one activity, e.g. 30 days maximum activity duration.
The second school of thought says to divide the project into small task subsets or activities. The advantage of this approach is that it provides more control of the schedule during execution. It is easy to tell the status of a small activity, such as install balance valve or install pump, and, therefore, making a status of these activities is relatively straight forward. The disadvantage of many small activities in defining a schedule is that it makes the Gantt chart more complex and confusing. This is why a third approach utilized by Primavera P6 is widely accepted, although you should always plan to the appropriate level of detail required to measure and control the project.
Primavera P6 activities in the Gantt chart are given relatively long durations to keep the Gantt chart simple. However, if appropriate, these activities can be further described using the Steps function in Primavera P6. The Steps function allows the scheduler to associate numerous smaller tasks associated with a lengthy duration activity on the Gantt chart. Additionally, these subtasks can be weighted among each other, which Primavera P6 uses to compute a percent weighting for each subtask.
Making a status of a Gantt chart activity is as simple as checking off the associated subtasks as complete. Primavera P6 will mark that portion of the corresponding activity as complete with the status bar. This gives the scheduler the advantages of both approaches; the simplicity of the first approach’s Gantt chart combined with the control of the second approach’s small activity definitions.
Control Factors to Consider
When defining activities on the Gantt chart or even with Primavera P6’s Steps function there are several factors the scheduler must consider. To break down individual activities to better control the schedule of the project consider:
1) The similarity or homogeneity of the work: Place similar work in the same basket. Are the tasks apart of demolition, excavation, or installation? These tasks should be grouped separately. The excavation work involved of digging a trench can be grouped separately from the mechanical work of laying the piping system in the trench.
2) Location of the work: A large building can be broken down into sections with all the mechanical and electrical work for these sections being grouped together.
3) Size of the work: Divide large activities with large quantities into small portions. Large excavation projects can be divided by grid or depth. Trenching excavation projects can be divided by 100-foot segments.
4) Timing/Chronology: Consider the chronology of the work being performed, e.g. demolition comes before installation work and also excavation before installation.
5) Responsibility: Work performed by different subcontractors should be considered separately.
6) Phases: The designer can break a large project into phases based on timing, location, or other factors.
Activity Duration Estimation
Once your schedule has been broken down into definable tasks it is essential to accurately estimate the length or duration of each of these tasks. RS Means can be a useful source for estimating the length of construction project tasks. RS Means is known for providing construction material, labor, and equipment costs. It also provides a description of the crews and their daily output. One simple formula that one can use in conjunction with RS Means is:
Duration = Total Quantity / Daily Output
For example, for a task that is 8,000 cubic yards of excavation with a crew that performs 500 cubic yards of excavation per day the activity duration equals 16 days duration for this excavation task. In addition to quantities, crew size, and crew productivity other factors that should be considered for activity duration are weather conditions, design complexity, and, possibly, other additional factors. Sometimes clearly defined factors are not available or sufficient for defining an activity duration. In these, hopefully, rare occasions the activity duration is subjective, and left to the respective electrical or mechanical crew to decide.
Of course, the project manager or technical engineer has input into these uncertain durations. Experience of similar past projects may also be helpful for determining durations. The scheduler must also consider non-work days, such as holidays or shutdowns. In the Mid-Atlantic, concrete pouring in the winter time is prohibitive, so this entails a shutdown of concrete pouring until warmer weather conditions arrive.
Defining activities and activity durations is important in developing a project schedule that is believable. The smaller the activity the greater the control. Too many small activities makes the Gantt chart difficult to read unless the scheduler is employing Primavera P6’s Steps function. Creating unique tasks is only one part of the equation for accurately modeling a schedule duration. The scheduler must also accurately estimate the duration of tasks.
For construction projects estimating books like RS Means can be very helpful to the scheduler. Other industries like the research industry will require more subjective estimates. On occasions where clearly defined units are not available it is recommended to be conservative in estimating activity durations. Once we have relatively accurate tasks broken down into activities and the corresponding durations, we can use the project scheduling software to determine the critical path or length of the project and track the status of work.