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Introduction:
The programme evaluation and review technique (PERT) is a model for project management designed to analyze and represent the tasks involving in completing a given project. It is commonly used in conjunction with critical path method or CPM. Critical path method is mathematically based algorithm for scheduling a set of project activities
Definition:
It is a method to analyze the involved tasks in completing a given project, especially the time needed to complete each task and identifying the minimum time needed to complete the total project.
PERT includes:
• The finished product or service desired
• The total time and budget needed to complete the project or program
• The starting date and completion date
• The sequence of steps or activities that will be required to accomplish the project or program
• The estimated time and cost of each step or activity
Eg:- PERT network chart for a seven month project with five milestones (10 through 50) and six activities(A through F)
Terminologies used in the PERT chart:
• PERT Event: A point that marks the start or completion of one or more activities. It consumes no time and uses no resources.
• Predecessor Event: An event that immediately precedes some other event without any other events intervening.
• Successor event: An event that immediately follows some other event without any other intervening events.
• PERT activity: The actual performance of a task which consumes time and requires resources
• Optimistic time (O): The minimum possible time required to accomplish a task.
• Pessimistic time (P): The maximum possible time required to accomplish a task,
• Expected time (TE): The best estimate of the time required to accomplish a task, assuming everything proceeds as normal
TE = (O + 4M + P) ÷ 6
• Most likely time (M): The best estimate of the time required to accomplish a task, assuming everything proceeds as normal.
• Float / slack: It is a measure of the excess time and resources available to complete a task. It is the amount of time that a project task can be delayed without causing a delay in any subsequent tasks (free float) or the whole project (total float). Positive slack would indicate ahead of schedule; negative slack would indicate behind schedule; and zero slack would indicate on schedule.
• Critical path: The longest possible continuous pathway taken from the initial event to the terminal event. It determines the total calendar time required for the project; and, therefore, any time delays along the critical path will delay the reaching of the terminal event by at least the same amount.
• Critical activity: An activity that has total float equal to zero. An activity with zero float is not necessarily on the critical path since its path may not be the longest.
• Lead time: The time by which a predecessor event must be completed in order to allow sufficient time for the activities that must elapse before a specific PERT event reaches completion.
• Lag time: The earliest time by which a successor event can follow a specific PERT event.
• Fast tracking: Performing more critical activities in parallel.
• Crashing critical path: Shortening duration of critical activities
Steps in the PERT planning process:
PERT involves the following steps
Identify the specific activities and mile stones
Determine the proper sequence of the activities
Construct a network diagram
Estimate the time required for each activity
Determine the critical path
Update the PERT chart as the project progresses
1. Identify the specific activities and mile stones
• The activities are the tasks required to complete the project
• The mile stones are the events marking the beginning and end of one or more activities
• It is helpful to list the tasks in the table that in later steps can be expanded to include information on sequence and duration.
2. Determine the proper sequence of the activities
• This step may be combined with the activity identification step since the activity sequence is evident for some tasks.
• Other tasks may require more analysis to determine the exact order in which they must be performed.
3. Construct the network diagram
• By using the activity sequence information a network diagram can be drawn.
• For the original activity on- arc model the activities are depicted by arrowed lines and milestones can be depicted by circles or bubbles.
4. Estimate activity times
A distinguishing feature of the PERT is its ability to deal with uncertainty
in activity completion times. For each activity the model usually includes three
time estimates.
• Optimistic time - Generally the shortest time in which the activity can be completed. It is common practice to specify optimistic times to be three standard deviations from the mean so that there is approximately a 1% chance that the activity will be completed within the optimistic time.
• Most likely time - The completion time having the highest probability. Note that this time is different from the expected time.
• Pessimistic time - The longest time that an activity might require. Three standard deviations from the mean is commonly used for the pessimistic time.
5. Determine the Critical Path
The critical path is determined by adding the times for the activities in each sequence and determining the longest path in the project. The critical path determines the total calendar time required for the project
The variance in the project completion time can be calculated by summing the variances in the completion times of the activities in the critical path. The four activities in this are:-
ES - Earliest Start time
EF - Earliest Finish time
LS - Latest Start time
LF - Latest Finish time
Since the critical path determines the completion date of the project, the project can be accelerated by adding the resources required to decrease the time for the activities in the critical path. Such a shortening of the project sometimes is referred to as project crashing.