PERT – Basic Concept
PERT was first used by NASA to manage their ventures and to prepare flow charts of their operations where the duration of activities are mostly guessed works and estimation of cost highly depends on such guess works.
PERT, stands for Program Evaluation and Review Technique, a practical display of a theoretical developed plan which is designed to investigate and show the task required for completing a project. The technique of PERT chart works together in complement with the concept of identifying the activity which can influence the project completion time this concept in project management is called the critical path technique, CPM.
PERT network mesh gives details about an activity involved in the project, besides that it gives suitable information about the interrelation of different activities and how long they would take to get completed. This chart comprises several nodes. There is no fixed number of nodes, however, as a rule of thumb two nodes one at the beginning known as initiating node and one at the end known as closing/ending node, form part of the PERT chart. If a various number of tasks form the initiation of the project then all the activities will begin from the initiating node. Every task is assigned with a code, and a line is drawn to connect it to the inheriting activity. This line is assigned a special coded name or the real name of the activity, and it gives information about the resources allocated to the activity and the duration required for the activity to get completed is also given. The other end of the line concludes at a new node representing the initiation of a new task, or the start of slack time, that is available or waiting duration among the tasks.
Every task representing a job is related to its inheritor in such a way that they form a mesh of nodes and connectors. The chart gets completed once all the final tasks join the completion node simultaneously. If the slack duration is there among the end of one job and start of another, then a hatched or dotted line is drawn between the endpoint of one job and initiating point of another job which initiates the dependant task immediately after this task. There is a high probability that different activities may be executed at the same time or different activities may follow one another in a sequential manner which can be shown on the PERT chart by showing parallel networks. There are some nodes where the project needs to be reviewed these critical points are known as milestones. PERT chart through smart action and intelligence ensures that major activities terminate on a review node for better project quality. It is worth noticing that the reviews, recommendations, and approvals take at least one to two weeks time and the common mistake made by most of the project planners is that they underestimate this time by keeping the minimum time for review activities. An appropriate time, say 1 to 2 weeks or more should be kept as it’s a time-consuming activity. The poorly managed project always underestimates the time required for documentation of the project works. It may appear unimportant initially but at later stages, one realizes its importance. Therefore the appropriate time for completing the documents of the project and compiling them should be kept in consideration.
PERT chart is drawn on special sized and special format paper in which the x-axis represents the duration. Though drawing a PERT chart for the complete project is possible but least recommended. Instead of drawing the PERT chart for the entire project, the project is broken down into small jobs and more influential parts. The main advantage of implementing this tactic is that it saves time because if there is an error due to an incorrectly estimated task, a need to draw the whole Chart again won’t arise. In fact, only a specific job’s chart will be redrawn.
Many construction companies follow the trend of ending the major tasks at the funding review node. This helps in determining which activity needs more funds, which activity can spare funds, which activity requires more human resources and from which activity human resources can be spared. Human resource certainly affects the pace of work. If such planning is not done then there is a chance that an activity may be having human resources more than required or an activity may be having human resources less than the required number. All such matters should be taken into account for a successful project plan and schedule.
PERT Planning Process
In the planning process, the points where data of activities need to be checked are identified, moreover, important activities which can turn out to be critical may be identified as well. The next step is to define a pattern in which activities should be executed for the timely completion of the project. After this step, the PERT chart is developed. Then a careful estimation of time for each activity is done. When the estimation is done then the path with max duration is identified and shown as a critical path. When the project is initiated then according to the progress in actual the PERT chart is constantly updated for better information control. (Donaldson and W. A, 1965)
The below figure shows the PERT chart to give a better understanding:
Advantages of PERT Chart
PERT chart clearly identifies and shows precedence relationships of elements of Work Breakdown Structure, WBS. PERT chart is an easy way of marking critical paths and critical activities merely by looking at the duration and sequence. It helps in defining the early start, late start and the duration of an activity can be delayed. Further PERT gives a better understanding of the precedence relationship which helps in overlapping or executing a job simultaneously which makes the reduction of project duration feasible. The resources from activities with idle time can be allocated to activities on the critical path to make sure the project is not delayed.
Disadvantages of PERT Chart
In real-time, there are hundreds of activities and individual precedence relationships. The network chart needs a special page setup and it is larger in size and may need several pages to print the network chart. Although different colors can help show the progress status but usually the time frame on most of the PERT charts is not mentioned which is a big disadvantage. The PERT charts are at times so much difficult to carry. Irrespective of how much care is taken while time estimations for the project activities are made, PERT will still assume beta distribution and this may result in a situation different than real. (Farnum, N. R. and Stanton, 1987). If any other activities in any path other than the critical path face delay in completion, it results in that path becoming critical. So due to this issue, more than one critical path may exist which will cause misunderstanding and ultimately wrong estimation of the time. (Schonberger, R. J, 1981)
Forward Pass
In forwarding pass, we determine early start (ES) and early finish (EF) times for each task. The calculations are made while working from left to right. Then the times in each path are added. It should be kept in mind that when a number of tasks end at a similar node then the ES for the next activity is the largest EF of the preceding activity.
Reverse Pass
In order to determine for how much duration an activity can be delayed in starting or finishing, we move in a reverse manner in contrast to forward pass. In this case, the end node becomes starting node. The bottom pair numbers are calculated in the backward pass. The durations are deduced from connecting nodes’ earliest start times.
PERT is used for planning large-scale complex projects with numerous interdependent tasks, a project which requires various activities, some of which need to be performed at the same time and some of which are to be performed one after another in a specific sequence.
Assumptions in PERT
The time estimates for the completion of every task are of equal importance. PERT starts with three possible assumptions those are optimistic, most likely and pessimistic.(MacCrimmon, K. R. and Ryavec, 1964)
Optimistic time(a)
According to assumptions of optimistic time, things will go according to the schedule with minimum difficulty. However, this is not close to a realistic approach as this may occur once in hundred times.
Most pessimistic time(b)
Most pessimistic time assumes that not everything planned will go accordingly and will assume that maximum potential difficulties may occur.
Most likely or normal time(m)
This is an assumption according to which thing would mostly occur if reported time and again.
The estimated time of the activity to be completed is given by:
- te= (a+4m+b)/6
Where:
- te = estimated time
- a = most optimistic time
- m = normal time
- b = most pessimistic time
Recent Studies and Research About PERT
Modern Studies reveal that when PERT is properly used then it effectively cut the project cost and reduces time. Along with this it also speeds up the planning process. It helps in eliminating the idle time that is slack time. Through such effective Project Management tools, one can give time to activities of priority in a much smart and effective manner. Project Managers can obtain extensive details by examining the network charts of the projects. Like a PERT chart will show how the activities are sequenced and which activity can occur simultaneously and which needs certain tasks to get completed before initiation of a particular task.
For difficult problems involving a number of activities, Computers are used to develop and examine the project schedule and network. Early start, Late Start, Early Finish Late Finish, is the value that is given to the computer as input. When the computer reads these values, its predefined perimeters and algorithm give information about the duration of the project, where and what resources are to be reduced or increased. Though computer algorithms are used however conclusion is that estimates should be good to get the best possible results. (Management Theory and Practice, 2005)
PERT is advantageous to Project Managers. For example, it helps them to sequence and sort the activities of the project and also give a diagrammatic view of how the activities of the project would take place and in what sequence. It also helps them to understand which activities are critical to the project completion duration and should be carefully observed, and which activities involve idle time and can be delayed without influencing the project completion duration. In a project with complex nature, there are various interdependencies involved and it makes it difficult to determine how things would occur in the future. The expertise of personnel in project management will definitely make things easy to understand and implement. (Keefer, D. L. and Verdini, 1993)
A recent report revealed the facts behind the failure of PERT in construction Management. The construction industry has inferred that PERT deviation from on-ground reality is due to its less closeness and information about processes involved in construction. This tool could not become famous in construction industries because the approach was nonrealistic and quite different from the way how things happen in reality. Due to the failure of techniques of Network many construction companies do not rely on PERT completely. These negative aspects make it difficult to project and promote PERT for schedule and network formation. If some contractors have to provide a PERT chart and schedule in the bid at the will of the client, they usually hire services of the third party and pay them to get things done in a suitable manner. So far, no matter how progress has been made in the construction industry still the use of project management software and employment of project managers is at a minimum because the construction industry so far could not adapt itself to this change. Very few realized the effectiveness of PERT. However, It is quite useful in projects where the requirement is neither a true model nor a process involved in construction. (Krajewski, L. J. and Ritzman, 2005),
In Construction each subcontractor is very keen on the smart use of his resources on all projects he is executing. A good project manager will efficiently manage resources according to the requirement and importance. Finishing earlier in some projects least matters whereas, in some projects finishing earlier can entitle you to a bonus. At the minimum, lowering cost is to be taken into account as much as an aim of reducing the entire duration.
Similar to the situation of medicine in which human bodies sometimes reject the transplanted organ from other humans who is not genetically connected to the patient, this should not be surprising that the construction industry has not accepted and incorporated the use of modern project managing tool into its functional stream completely because of the high possibility of errors that may occur. The concepts of PERT, despite rewarding and fruitful in other industries, are alien to the process involved in construction which developed its processes over years and got evolved to this state and satisfies the need of construction steps involved. Construction Industry believes in an approach that manually scheduling things is though time-consuming but still, the chances of error and underestimation leading to cost and time constrain and cost impact is minimized in comparison to those schedules devised by computers by using PERT as there is a bright chance of beta distribution and biased distribution of time and there is a chance that critical path selected by computer is not the really critical one.
References
Donaldson, W. A. (1965), “The Estimation of the Mean and Variance of a ‘PERT’ Activity Time,” Operations Research, Vol. 13, No. 3, pp. 382-385.
Farnum, N. R. and L. W. Stanton (1987), “Some Results Concerning the Estimation of Beta Distribution Parameters in PERT,” The Journal of the Operational Research Society, Vol. 38, No. 3, pp. 287-290.
Heizer, J. H. and B. Render (2004), Operations Management, Pearson-Prentice Hall, Upper Saddle River, NJ.
Keefer, D. L. and W. A. Verdini (1993), “Better Estimation of PERT Activity Time Parameters,” Management Science, Vol. 39, No. 9, pp. 1086-1091.
Krajewski, L. J. and L. P. Ritzman (2005), Operations Management: Processes and Value Chains. Pearson/Prentice Hall. Upper Saddle River, NJ.
MacCrimmon, K. R. and C. A. Ryavec (1964), “An Analytical Study of the PERT Assumptions,” Operations Research, Vol. 12, No. 1, pp. 16-37.
Management Theory and Practice. (2005)” Omega The International Journal of Management Science, Vol. 17, No.1, pp. 21-25.
Schonberger, R. J. (1981), “Why Projects Are “Always” Late: A Rationale Based on Manual Simulation of a PERT/CPM Network,” Interfaces, Vol. 11, No. 5, pp. 66-70.