Pert Charts Essay, Research Paper
A PERT chart is a project management tool used to schedule, organize, and coordinate tasks within a project. PERT stands for Program Evaluation Review Technique, a methodology developed by the U.S. Navy in the 1950s to manage the Polaris submarine missile program. A similar methodology, the Critical Path Method (CPM), which was developed for project management in the private sector at about the same time, has become synonymous with PERT, so that the technique is known by any variation on the names: PERT, CPM, or PERT/CPM.
A PERT chart presents a graphic illustration of a project as a network diagram consisting of numbered nodes (either circles or rectangles) representing events, or milestones in the project linked by labeled vectors (directional lines) representing tasks in the project. The direction of the arrows on the lines indicates the sequence of tasks. In the diagram, for example, the tasks between nodes 1, 2, 4, 8, and 10 must be completed in sequence. These are called dependent or serial tasks. The tasks between nodes 1 and 2, and nodes 1 and 3 are not dependent on the completion of one to start the other and can be undertaken simultaneously. These tasks are called parallel or concurrent tasks. Tasks that must be completed in sequence but that don’t require resources or completion time are considered to have event dependency. These are represented by dotted lines with arrows and are called dummy activities. For example, the dashed arrow linking nodes 6 and 9 indicates that the system files must be converted before the user test can take place, but that the resources and time required to prepare for the user test (writing the user manual and user training) are on another path. Numbers on the opposite sides of the vectors indicate the time allotted for the task.
Each task is connected to its successor tasks in this manner forming a network of nodes and connecting lines. The chart is complete when all final tasks come together at the completion node. When slack time exists between the end of one task and the start of another, the usual method is to draw a broken or dotted line between the end of the first task and the start of the next dependent task.
A PERT chart may have multiple parallel or interconnecting networks of tasks. If the scheduled project has milestones, checkpoints, or review points (all of which are highly recommended in any project schedule), the PERT chart will note that all tasks up to that point terminate at the review node.
The critical path = total time for activities on this path is greater than any other path through the network (delay in any task on the critical path leads to a delay in the project).
Critical Path Method (CPM) charts are similar to PERT charts and are sometimes known as PERT/CPM. In a CPM chart, the critical path is indicated. A critical path consists that set of dependent tasks (each dependent on the preceding one) which together take the longest time to complete. Although it is not normally done, a CPM chart can define multiple, equally critical paths. Tasks which fall on the critical path should be noted in some way, so that they may be given special attention. One way is to draw critical path tasks with a double line instead of a single line.
Tasks which fall on the critical path should receive special attention by both the project manager and the personnel assigned to them. The critical path for any given method may shift as the project progresses; this can happen when tasks are completed either behind or ahead of schedule, causing other tasks which may still be on schedule to fall on the new critical path.
The PERT chart is sometimes preferred over the Gantt chart, another popular project management charting method, because it clearly illustrates task dependencies. On the other hand, the PERT chart can be much more difficult to interpret, especially on complex projects.
Tasks on the critical path therefore need to be monitored carefully.
Drawing a PERT diagram
+ There is a single start and end event;
+ Time flows from left to right (so does the numbering sequence);
+ Events are given a unique number (activities then have a unique label i.e. head & tail event numbers);
+ The network can then be drawn taking into account the dependencies identified;
+ Working from the start event forward, calculate the earliest times, setting the earliest time of the first event to zero. Add the job duration time to the earliest event time to arrive at the earliest time for the successor event. Where the successor has more than one activity dependent on to the latest time is entered;
+ Working from the finish event backwards, calculate the latest times. Set the latest time to the earliest time for the finish event. Subtract job duration from the latest time to obtain predecessor latest event times. Where the predecessor event has more than one arrow emanating from it enter the earliest time;
+ Event slack is calculated by subtracting the earliest event time from the latest event time;
+ Critical path(s) are obtained by joining the events with zero event slack.
1. Setting the project start date
2. Setting the project completion date
3. Selecting the project methodology or project life cycle to be used
4. Determining the scope of the project in terms of the phases of the selected project methodology or project life cycle
5. Identifying or selecting the project review methods to be used
6. Identifying any predetermined interim milestone or other critical dates which must be met.
7. Listing tasks, by project phase, in the order in which they might be accomplished.
8. Estimating the personnel necessary to accomplish each task
9. Estimating the personnel available to accomplish each task
10. Determining skill level necessary to perform each task
11. Determining task dependencies
a. Which tasks can be done in parallel
b. Which tasks require the completion of other tasks before they can start
12. Project control or review points
13. Performing project cost estimation and cost-benefit analysis