This article will discuss the fourth project constraint — time. After determining that the project is profitable, accepting it, decomposing the work scope into smaller tasks and assigning responsibilities, it is necessary to schedule the project — i.e. determine the amount of time needed to complete all project tasks. The time analysis is critical for two reasons. First, there will be urgent projects with tight deadlines (which are usually more profitable). The second is that not finishing the project in the agreed timeframe is considered unprofessional.
The best methods for project time analysis are Program evaluation and review technique (PERT) and Critical path method (CPM).
Program Evaluation and Review Technique (PERT)
PERT is a method used to analyze project tasks and time needed to complete them. It was developed for the U.S. Navy Special Projects Office in 1957 to support the U.S. Navy's Polaris nuclear submarine project.1 The PERT method is usually used in projects where time is a more important factor than cost and is convenient for large and complex projects.
The first step of the PERT method is to identify the project tasks and the order in which they need to be completed (this can be done using the work breakdown structure, as discussed in the previous article). Some tasks can be done simultaneously, while others cannot be done until one or more previous tasks are complete. Also, each task has three time estimates: optimistic time, normal time and pessimistic time estimate. The optimistic time is the fastest possible time needed to complete the task and usually involves additional cost. Normal time is the most likely time, while the pessimistic time is the slowest time needed to complete the task. Using these time estimates, the expected time can be calculated with the following formula:
- Te — Expected time
- O — Optimistic time
- M — Normal time
- P — Pessimistic time
The estimates and calculations are written in a table. Here is an example with seven tasks (ordered A-G):
The data from the table can be used to draw a Gantt chart or a network diagram.
Critical Path Method (CPM)
Critical path method is a project modeling technique invented at roughly the same time as the PERT method. CPM is commonly used with all forms of projects, including construction, aerospace and defense, software development, research projects, product development, engineering, and plant maintenance, among others. Any project with interdependent activities can apply this method of mathematical analysis.2
Using the data from the PERT method (the list of the project tasks, dependencies between them and the time required to complete each task), the CPM determines the longest path to the end of the project. This path is called the critical path and represents the sequence of project network activities that add up to the longest overall duration. The critical path is the shortest time possible to complete the project. Any delay of an activity on a critical path will delay the completion of the entire project. The activities that are not on the critical path can be delayed without affecting the time required to complete the project.
The critical path is usually presented as a network diagram (activity-on-node diagram), which can be drawn by hand or using the diagram software. The diagram usually starts with a node named "start", which has the duration of zero. Then the activities without predecessor activities are drawn and the start node is connected to them with arrows. If we take the data from the PERT table above as an example, these activities would be A and B. Other activities are drawn in a similar fashion. Finally, the node named "finish" is created and the activities that don't have any activities after them (in our case F and G) are connected to it. Each node on the diagram usually contains the following information:
- The activity name
- The normal duration time
- The early start time (ES)
- The early finish time (EF)
- The late start time (LS)
- The late finish time (LF)
The ES is defined as the maximum EF of all predecessor activities, unless the activity in question is the first activity, for which the ES is zero (0). The EF is the ES plus the task duration (EF = ES + duration). The LF is defined as the minimum LS of all successor activities, unless the activity is the last activity, for which the LF equals the EF. The LS is the LF minus the task duration (LS = LF − duration).3
Differences Between PERT and CPM
Although the two time analysis methods are very similar, PERT and CPM have a few notable differences. The most important one is that in PERT, the time is probabilistic (the most likely time), while in the CPM the time is deterministic (exact number). Also, PERT is used in projects that have unpredictable tasks, such as research and development projects, while the CPM is used in projects with predictable tasks, such as construction projects. Furthermore, cost optimization is the primary goal of CPM, while PERT minimizes the time needed to complete the project.
Time Management Tips
- Identify project tasks and order in which they need to be completed
- Decide which time analysis method will you use, depending on the project type
- If you are using PERT method, estimate optimistic, normal and pessimistic time and calculate the expected time
- Present the data as a table for easy reading
- Draw a Gantt chart or a network diagram
- The network diagram is drawn the same way for both PERT and CPM methods. The only difference is that PERT uses the expected time (calculated from three time estimates), while the CPM makes only one time estimate for each task
More Articles in This Series
1. [Malcolm, D. G., J. H. Roseboom, C. E. Clark, W. Fazar "Application of a Technique for Research and Development Program Evaluation OPERATIONS RESEARCH Vol. 7, No. 5, September–October 1959, pp. 646–669"] ↩
2. ["Critical path method", Wikipedia, Retrieved February 24, 2015, from http://en.wikipedia.org/wiki/Critical_path_method] ↩
3. ["Program evaluation and review technique", Wikipedia, Retrieved February 24, 2015, from http://en.wikipedia.org/wiki/Program_evaluation_and_review_technique] ↩