This article gives you a holistic idea about two popular project management techniques: PERT and CPM. In this article, you will learn about the significance, uses, methodology of the techniques, and role of Gantt Chart in the project management.
- Common Terms
- Pre Project Activity
- Significance of CPM/PERT
- Gantt Charts:
- Brief History of PERT and CPM
- Framework for PERT and CPM
- Drawing the CPM/PERT Network
Program Evaluation and Review Technique
Critical Path Method
Basically, PERT, CPM are the 2 popular project management techniques, which have been created out of the need of Western industrial and military establishments to plan, schedule and control complex projects.
To understand the whole concept of these techniques, we need to know some important terms that are used,
The longest complete path of a project.
A single task along a critical path
Something of value generated by a team or individual as scheduled often taking the form of a plan, report, procedure, product, or service.
A task or subtask that cannot be initiated until a predecessor task or several predecessor tasks are finished.
A link that shows an association or relationship between two otherwise parallel tasks along a PERT/CPM network.
A significant event or juncture in the project.
A task within a CPM network for which slack time is available.
Two or more tasks that can be undertaken at the same time. This does not imply that they have the same starting and ending times.
A chronological sequence of tasks, each dependant on the predecessors.
Task that must be completed before another task can be completed.
The allocation of resources over a specific timeframe and the coordination of interrelated events to accomplish an overall objective while meeting both predictable and unique challenges.
A critical project element such as money, time, or human resources.
Scope of the project or scope of the project
The level of activity and effort necessary to complete a project and achieve the desired outcomes as measured by hours, days, resources consumed, and funds spent.
Margin or extra room to accommodate anticipated potential short falls in planning
The time interval in which you have leeway as to when a particular task needs to be completed.
Task or Event
A divisible, definable unit of work related to a project, which may or may not include subtasks.
The scheduled start and stop times for a subtask, task, phase or entire project.
Pre Project Activity
Before attempting to use or know about these tools, the project’s information must be assembled in a certain way. It includes a basic description of the preceding steps.
- Setting the project start date
- Setting the project completion date
- electing the project methodology or project life cycle to be used
- Determining the scope of the project in terms of the phases of the selected project methodology or project life cycle
- Identifying or selecting the project review methods to be used
- Identifying any predetermined interim milestone or other critical dates which must be met.
- Listing tasks, by project phase, in the order in which they might be accomplished.
- Estimating the personnel necessary to accomplish each task
- Estimating the personnel available to accomplish each task
- Determining skill level necessary to perform each task
- Determining task dependencies
- Which tasks can be done in parallel?
- Which tasks require the completion of other tasks before they can start?
- Project control or review points
- Performing project cost estimation and cost-benefit analysis
Significance of CPM/PERT
There are many variations of CPM/PERT which have been useful in planning costs, scheduling manpower and machine time. The main significance of using CPM/PERT is that, they answer the following important questions of a project,
- How long will the entire project take to be completed? What are the risks involved?
- Which are the critical activities or tasks in the project which could delay the entire project if they were not completed on time?
- Is the project on schedule, behind schedule or ahead of schedule?
- If the project has to be finished earlier than planned, what is the best way to do this at the least cost?
Answer to these question prior to the start of project gives an in-detailed idea of the project and foreseen problems that the project will or can face in the future.
Before we know about CPM or PERT, there is an import tool or method that is used to easen the work known as GANTT CHART
Henry Gantt who the Gantt chart is named, worked for the department of defense during the First World War. The chart is widely used as a project management tool. The Gantt chart allows you to see start and stop date for project task and subtask. Gantt Charts are derived from your Work Breakdown Structure (WBS)
Work breakdown Structures
The development of a project plan is predicated on having a clear and detailed understanding of both the tasks involved, the estimated length of time each task will take, the dependencies between those tasks, and the sequence in which those tasks have to be performed. Additionally, resource availability must be determined in order to assign each task or group of tasks to the appropriate worker. One method used to develop the list of tasks is to create what is known as a work breakdown structure.
A work breakdown structure (WBS) is a hierarchic decomposition or breakdown of a project or major activity into successive levels, in which each level is a finer breakdown of the preceding one. In final form a WBS is very similar in structure and layout to a document outline. Each item at a specific level of a WBS is numbered consecutively (e.g., 10, 10, 30, 40, 50). Each item at the next level is numbered within the number of its parent item (e.g., 10.1, 10.2, 10.3, 10.4).
The WBS may be drawn in a diagrammatic form (if automated tools are available) or in a chart resembling an outline.
The WBS begins with a single overall task representing the totality of work to be performed on the project. This becomes the name of the project plan WBS. Using a methodology or system life cycle (analysis, design and implementation) steps as a guide, the project is divided into its major steps. The first phase is project initiation; the second major phase is analysis, followed by design, construction, testing, implementation, and post-implementation follow-up. Each of these phases must be broken in their next level of detail, and each of those, into still finer levels of detail, until a manageable task size is arrived at. The first WBS level for the life cycle would be:
WBS Number Task Description
1.0 Project initiation
1.1 Draft project plan
2.0 Analysis phase
2.1 Plan user interviews
2.2 Schedule users interviews
3.0 Examination and test
Tasks at each successively finer level of detail are numbered to reflect the task from which they were derived. Thus, the first level of tasks would be numbered 1.0, 2.0, 3.0, and so forth. Each of their subtasks would have a two-part number: the first part reflecting the parent task and the second part, the subtasks number itself, such as 1.1, 1.2, or 1.3. As each of these, in turn, decomposed or broken down into its component tasks, each component receives a number comprised of its parent number plus a unique number of its own.
Brief History of PERT and CPM
CPM was the discovery of M.R.Walker of E.I.Du Pont de Nemours & Co. and J.E.Kelly of Remington Rand, circa 1957. The computation was designed for the UNIVAC-I computer. The first test was made in 1958, when CPM was applied to the construction of a new chemical plant. Unproductive time was reduced from 125 to 93 hours.
PERT was devised in 1958 for the POLARIS missile program by the Program Evaluation Branch of the Special Projects office of the U.S.Navy, helped by the Lockheed Missile Systems division and the Consultant firm of Booz-Allen & Hamilton.
Framework for PERT and CPM
Essentially, there are six steps which are common to both the techniques. The procedure is listed below:
- Define the Project and all of its significant activities or tasks. The Project (made up of several tasks) should have only a single start activity and a single finish activity.
- Develop the relationships among the activities. Decide which activities must precede and which must follow others.
- Draw the “Network” connecting all the activities. Each Activity should have unique event numbers. Dummy arrows are used where required to avoid giving the same numbering to two activities.
- Assign time and/or cost estimates to each activity
- Compute the longest time path through the network. This is called the critical path.
- Use the Network to help plan, schedule, monitor and control the project.
The Key Concept used by CPM/PERT is that a small set of activities, which make up the longest path through the activity network control the entire project. If these “critical” activities could be identified and assigned to responsible persons, management resources could be optimally used by concentrating on the few activities which determine the fate of the entire project.
Non-critical activities can be replanned, rescheduled and resources for them can be reallocated flexibly, without affecting the whole project.
Five useful questions to ask when preparing an activity network are:
- Is this a Start Activity?
- Is this a Finish Activity?
- What Activity Precedes this?
- What Activity Follows this?
- What Activity is Concurrent with this?
Some activities are serially linked. The second activity can begin only after the first activity is completed. In certain cases, the activities are concurrent, because they are independent of each other and can start simultaneously. This is especially the case in organisations which have supervisory resources so that work can be delegated to various departments which will be responsible for the activities and their completion as planned.
When work is delegated like this, the need for constant feedback and co-ordination becomes an important senior management pre-occupation.
Drawing the CPM/PERT Network
Each activity (or sub-project) in a PERT/CPM Network is represented by an arrow symbol. Each activity is preceded and succeeded by an event, represented as a circle and numbered.
At Event 3, we have to evaluate two predecessor activities – Activity 1-3 and Activity 2-3, both of which are predecessor activities. Activity 1-3 gives us an Earliest Start of 3 weeks at Event 3. However, Activity 2-3 also has to be completed before Event 3 can begin. Along this route, the Earliest Start would be 4+0=4. The rule is to take the longer (bigger) of the two Earliest Starts. So the Earliest Start at event 3 is 4.
Similarly, at Event 4, we find we have to evaluate two predecessor activities – Activity 2-4 and Activity 3-4. Along Activity 2-4, the Earliest Start at Event 4 would be 10 wks, but along Activity 3-4, the Earliest Start at Event 4 would be 11 wks. Since 11 wks is larger than 10 wks, we select it as the Earliest Start at Event 4.
We have now found the longest path through the network.
It will take 11 weeks along activities 1-2, 2-3 and 3-4.
This is the Critical Path.
A manageable task is one in which the expected results can be easily identified; success, failure, or completion of the task can be easily ascertained; the time to complete the task can be easily estimated; ant the resource requirements of the task can be easily determined.
Program evaluation and review technique (PERT) charts depict task, duration, and dependency information. Each chart starts with an initiation node from which the first task, or tasks, originates. If multiple tasks begin at the same time, they are all started from the node or branch, or fork out from the starting point. Each task is represented by a line, which states its name or other identifier, its duration, the number of people assigned to it, and in some cases the initials of the personnel assigned. The other end of the task line is terminated by another node, which identifies the start of another task, or the beginning of any slack time, that is, waiting time between tasks.
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. It should be noted at this point that the project review, approvals, user reviews, and so forth all take time. This time should never be underestimated when drawing up the project plan. It is not unusual for a review to take 1 or 2 weeks. Obtaining management and user approvals may take even longer.
When drawing up the plan, be sure to include tasks for documentation writing, documentation editing, project report writing and editing, and report reproduction. These tasks are usually time-consuming; so don’t underestimate how long it will take to complete them.
PERT charts are usually drawn on ruled paper with the horizontal axis indicating time period divisions in days, weeks, months, and so on. Although it is possible to draw a PERT chart for an entire project, the usual practice is to break the plans into smaller, more meaningful parts. This is very helpful if the chart has to be redrawn for any reason, such as skipped or incorrectly estimated tasks.
Many PERT charts terminate at the major review points, such as at the end of the analysis. Many organizations include funding reviews in the projects life cycle. Where this is the case, each chart terminates in the funding review node.
Funding reviews can affect a project in that they may either increase funding, in which case more people have to make available, or they may decrease funding, in which case fewer people may be available. Obviously more or less people will affect the length of time it takes to complete 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.
A Gantt chart is a matrix, which lists on the vertical axis all the tasks to be performed. Each row contains a single task identification, which usually consists of a number and name. The horizontal axis is headed by columns indicating estimated task duration, skill level needed to perform the task, and the name of the person assigned to the task, followed by one column for each period in the project’s duration. Each period may be expressed in hours, days, weeks, months, and other time units. In some cases it may be necessary to label the period columns as period 1, period 2, and so on.
The graphics portion of the Gantt chart consists of a horizontal bar for each task connecting the period start and period ending columns. A set of markers is usually used to indicate estimated and actual start and end. Each bar on a separate line, and the name of each person assigned to the task is on a separate line. In many cases when this type of project plan is used, a blank row is left between tasks. When the project is under way, this row is used to indicate progress, indicated by a second bar, which starts in the period column when the task is actually started and continues until the task is actually completed. Comparison between estimated start and end and actual start and end should indicate project status on a task-by-task basis.
Variants of this method include a lower chart, which shows personnel allocations on a person-by-person basis. For this section the vertical axis contains the number of people assigned to the project, and the columns indicating task duration are left blank, as is the column indicating person assigned. The graphics consists of the same bar notation as in the upper chart indicates that the person is working on a task. The value of this lower chart is evident when it shows slack time for the project personnel, that is, times when they are not actually working on any project.