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Eleven important differences betweeb PERT and CPM are discussed in this article. One such difference is PERT is a technique of planning and control of time. Unlike CPM, which is a method to control costs and time.
Project management can be understood as a systematic way of planning, scheduling, executing, monitoring, controlling the different aspects of the project, so as to attain the goal made at the time of project formulation. PERT and CPM are the two network-based project management techniques, which exhibit the flow and sequence of the activities and events. Program (Project) Management and Review Technique (PERT) is appropriate for the projects where the time needed to complete different activities are not known.
On the other hand, the Critical Path Method or CPM is apt for the projects which are recurring in nature.
The two scheduling methods use a common approach for designing the network and for ascertaining its critical path. They are used in the successful completion of a project and hence used in conjunction with each other. Nevertheless, the truth is that CPM is different from PERT in a way that the latter concentrates on time while the former stresses on the time-cost trade-off. In the same manner, there are many differences between PERT and CPM, which we are going to discuss in this article.
Content: PERT Vs CPM
|BASIS FOR COMPARISON||PERT||CPM|
|Meaning||PERT is a project management technique, used to manage uncertain activities of a project.||CPM is a statistical technique of project management that manages well defined activities of a project.|
|What is it?||A technique of planning and control of time.||A method to control cost and time.|
|Evolution||Evolved as Research & Development project||Evolved as Construction project|
|Model||Probabilistic Model||Deterministic Model|
|Focuses on||Time||Time-cost trade-off|
|Estimates||Three time estimates||One time estimate|
|Appropriate for||High precision time estimate||Reasonable time estimate|
|Management of||Unpredictable Activities||Predictable activities|
|Nature of jobs||Non-repetitive nature||Repetitive nature|
|Critical and Non-critical activities||No differentiation||Differentiated|
|Suitable for||Research and Development Project||Non-research projects like civil construction, ship building etc.|
|Crashing concept||Not Applicable||Applicable|
Definition of PERT
PERT is an acronym for Program (Project) Evaluation and Review Technique, in which planning, scheduling, organizing, coordinating and controlling uncertain activities take place. The technique studies and represents the tasks undertaken to complete a project, to identify the least time for completing a task and the minimum time required to complete the whole project. It was developed in the late 1950s. It is aimed to reduce the time and cost of the project.
PERT uses time as a variable which represents the planned resource application along with performance specification. In this technique, first of all, the project is divided into activities and events. After that proper sequence is ascertained, and a network is constructed. After that time needed in each activity is calculated and the critical path (longest path connecting all the events) is determined.
Definition of CPM
Developed in the late 1950s, Critical Path Method or CPM is an algorithm used for planning, scheduling, coordination and control of activities in a project. Here, it is assumed that the activity duration is fixed and certain. CPM is used to compute the earliest and latest possible start time for each activity.
The process differentiates the critical and non-critical activities to reduce the time and avoid the queue generation in the process. The reason for the identification of critical activities is that, if any activity is delayed, it will cause the whole process to suffer. That is why it is named as Critical Path Method.
In this method, first of all, a list is prepared consisting of all the activities needed to complete a project, followed by the computation of time required to complete each activity. After that, the dependency between the activities is determined. Here, ‘path’ is defined as a sequence of activities in a network. The critical path is the path with the highest length.
Key Differences Between PERT and CPM
The most important differences between PERT and CPM are provided below:
- PERT is a project management technique, whereby planning, scheduling, organising, coordinating and controlling uncertain activities are done. CPM is a statistical technique of project management in which planning, scheduling, organising, coordination and control of well-defined activities take place.
- PERT is a technique of planning and control of time. Unlike CPM, which is a method to control costs and time.
- While PERT is evolved as a research and development project, CPM evolved as a construction project.
- PERT is set according to events while CPM is aligned towards activities.
- A deterministic model is used in CPM. Conversely, PERT uses a probabilistic model.
- There are three times estimates in PERT, i.e. optimistic time (to), most likely time ™, pessimistic time (tp). On the other hand, there is only one estimate in CPM.
- PERT technique is best suited for a high precision time estimate, whereas CPM is appropriate for a reasonable time estimate.
- PERT deals with unpredictable activities, but CPM deals with predictable activities.
- PERT is used where the nature of the job is non-repetitive. In contrast to, CPM involves the job of repetitive nature.
- There is a demarcation between critical and non-critical activities in CPM, which is not in the case of PERT.
- PERT is best for research and development projects, but CPM is for non-research projects like construction projects.
- Crashing is a compression technique applied to CPM, to shorten the project duration, along with the least additional cost. The crashing concept is not applicable to PERT.
Video: PERT Vs CPM
The difference between these two project management tools is getting blurred as the techniques are merged with the passage of time. That is why, in most projects, they are being used as a single project. The primary point that distinguishes PERT from CPM is that the former gives the extreme importance of time, i.e. if the time is minimized, consequently the cost will also be reduced. However, cost optimization is the basic element, in the latter.
Over many years of my training experience, I have found that people are confused with the following project management terminologies. I have put my best effort to clarify these terms.
Over many years of my training experience, I have found that people are confused with the following project management terminologies. I have put my best effort to clarify these terms. Please do share your feedback and comments for the benefits of the others.
1. Lead and Lag are the duration unit of tasks. Lead is portrayed by negative (-) sign whereas Lag is by positive (+) sign. A lead is the amount of time whereby a successor activity can be advanced with respect to a predecessor activity whereas A lag is the amount of time whereby a successor activity will be delayed with respect to a predecessor activity.
Lead and Lag are decided by the Project manager based on the logical relationship between two tasks. For example, Hang a photo frame on the wall. In this case, consider you have the following tasks…
a. Task A – Construct a wall
b. Task B – Paint a Wall
c. Task C – Hang a photo frame
Here, Task B is a predecessor to Task C and it also required waiting time (Lag) before the successor task can start. Now the lag will be decided by the project manager based on expert judgment and other facilitation techniques in this case.
Hence, Lead and Lag are always imposed by the project managers on a critical or non-critical path based on task logical relationship.
2. Float or Slack is again an activity duration unit where a task can be delayed without having impact on early start date of successor task (free float) or project duration (Total Float) or customer constraints date (Project float) but the difference here, it is not imposed by project manager manually rather they exist in network diagram because of the sequencing of the tasks.
Float or Slack can only exist on the non-critical path.
Formula: Float = LS-ES or LF – EF
3. Buffer or Reserve Or Padding are used in project management as a risk response strategy. They are again manually decided by project managers based on expert judgment or other facilitation techniques with the team and other stakeholders.
In the Project network diagram on the critical path, when we put buffer (Project Buffer) at the end of the network, it is for the purpose of mitigating the unknown-unknown risk and hence to protect the project schedule for delay from unidentified risk.
When we put Buffer on a non-critical path (Feeding Buffer) it is again for risk of delay to task on the non-critical path which has an impact on the task on the critical path because of their relationship/constraints.
To summarize, Float, Lead/Lag and Buffer are not the same. Float for the task cannot be decided by project manager whereas Lead/Lag and Buffer can be decided by him with other’s help.
Lead/Lag can exist on both critical or non-critical paths the same as Buffer but the difference is Buffer can also exist at the end of the critical path as oppose to Lead/Lag.
A Precedence Diagramming Method (PDM) is a graphical representation technique. It shows the inter-dependencies of activities and is used in schedule
A Precedence Diagramming Method (PDM) is a graphical representation technique. It shows the inter-dependencies of activities and is used in schedule development. The other name for this technique is Activity on Node (AON).
The Precedence Diagramming Method shows activity relationships. Hence, it is an important communication tool for stakeholders.
The Precedence Diagramming Method is made of rectangles, known as nodes. These boxes show the project activities. An arrow connects two boxes and shows the relationship. Therefore, these diagrams are also known as Activity on Node (AON) diagrams.
Type of Dependencies in PDM
The PDM uses four dependencies:
- Mandatory Dependency
- Discretionary Dependency
- External Dependency
- Internal Dependency
This dependency is also known as hard logic. You cannot avoid it. Starting the next activity depends on it.
For example, you cannot install the ceiling until you build all the walls.
This dependency is also known as preferential or soft logic, it plays a role in optimizing resources.
For example, you can construct the four walls in any sequence. However, if constructing them in a certain sequence is beneficial you build them in that order.
Here, you can change the sequence of activities as per your preferred logic.
The project management team has no control over an external dependency.
For example, you may need government approval before starting the next activity.
These are dependencies are within the control of your project or organization.
For example, you cannot get a resource until it is free from another project.
The Precedence Diagramming Method uses four relationships:
- Finish to Start (FS)
- Finish to Finish (FF)
- Start to Start (SS)
- Start to Finish (SF)
Finish to Start (FS)
Here, the next activity cannot start until the first is complete. This is the most common relationship in PDM.
For example, to paint a wall you first you have to construct it. In this case, the first activity is building the wall and the second activity is painting. You cannot start painting the wall until the wall is ready.
Finish to Finish (FF)
Here, you cannot complete the next activity until the first is finished. Put simply, both activities should be finished simultaneously.
For example, let us say that you are coding a program for a client. The client gives you the features after completing a milestone. You cannot finish coding until you get the client’s complete requirements. Here, both activities should be finished simultaneously.
Start to Start (SS)
Here, the next activity cannot be started until the first starts. Both activities should start simultaneously.
Suppose you have to apply a coating on a wall, but the wall must be cleaned in order to apply it.
Therefore, one team will clean the wall and second team will coat it. Both activities can start simultaneously.
Start to Finish (SF)
Here, you cannot finish the next activity until the first starts.
For example, let us say you are moving into a new home and your old home has to be demolished. In this case, you cannot move into your new home until it is ready. Hence, the second activity (construction of the new home) must be finished before the first activity starts (moving into a new home).
Put simply, you are moving into your new home. You cannot start vacating your old home until the new house is ready.
Although this relationship is rare, you must understand all the dependencies. It will help you draw the network diagram and develop the project schedule.
This concludes the precedence diagram method.
You may hear the term Activity on Arrow (AOA). This is a less commonly used technique in diagramming methods. The AOA method is a special case of the Precedence Diagramming Method.
AOA diagram only uses the Finish to Start relationship. It shows the duration over the arrows, that is why many experts call this diagramming method the Activity on Arrow diagram. PERT is an example of this technique.
There is a difference between the AON and AOA diagram. The AOA diagram emphasizes milestones (events), and the AON diagram emphasizes tasks.
How to Draw a Precedence Diagram
To draw a PDM, you will break your Work Breakdown Structure down to the activity level.
Then you will create a table, list all activities, and sequence the activities.
The next step is to add relationships to each activity. You will add what activity comes next.
Finally, you will draw the diagram.
Benefits of Precedence Diagramming Method
This method offers many benefits to project management:
- It helps you find relationships and dependencies among activities. This helps you in planning and avoiding risks. If any task is missing, you can easily identify it.
- You can find critical activities and focus on them. Any delay in critical activities will delay your schedule.
- A project schedule network diagram is a good communication tool. Stakeholders can visualize activities and understand the schedule.
- Without the Precedence Diagram, you cannot develop your project schedule.
The Precedence Diagram Technique has an important role in project management. Your project schedule depends on it and it is a good communication tool. It is commonly referred to as AON, where nodes represent activities. The other PDM is AOA, where nodes represent milestones and duration is shown on the arrow.
This topic is important for the PMP exam. You may see one or two questions on this topic.
- Statistical method for identifying factors which may influence a product or the processes
- May be used in quality to determine type and number of tests to be executed in control
- Represents a manner in which many product and process factors can be examined as opposed to attacking one at a time
- Considers cost impact on changes in processes
When it comes to quality planning, there are several tools that can be used by project managers and one of them is the design of experiments (DOE). It is a technique that is used to identify different factors that can influence different aspects of the process or product during the time when it is being developed. It is used during the quality planning management to determine the type of tests and the different impacts they have on the cost of quality.
Critical Path vs Critical Chain
Critical path and critical chain are both methods of project scheduling.
The critical path method calculates the early start and finish dates and late start and finish dates for all schedule activities.
The critical path method involves performing a forward pass analysis and a backward pass analysis through the project schedule network paths. The forward pass determines the early start (ES) and early finish (EF) dates (ES + duration (DU) = EF). The backward pass determines the late start (LS) and late finish (LF): (LF – DU = LS).
The critical path is the longest path through the schedule with either zero or negative total float.
Critical activities are those schedule activities on the critical path. Near-critical activities are those schedule activities with very little total float.
The critical chain method is a technique that modifies the project schedule to account for limited resources by adding duration buffers that are non-work schedule activities to maintain focus on the planned activity durations.
Critical chain is completed after determining critical path by entering resource availability and the resulting schedule produces a resource-constrained critical path, which is usually altered from the original. Critical chain focuses on managing remaining buffer durations against the remaining durations of task chains.
The critical path method (CPM) is a popular approach to project scheduling that considers the amount of float on project activities. Critical chain takes CPM a step further by adding time buffers to account for limited resources.
Rolling wave planning is a project management technique that involves progressive elaboration to add detail to the Work Breakdown Structure (WBS) on an ongoing basis. At the beginning of the project, near term deliverables are decomposed into individual components (work packages) and defined at the greatest level of detail. Deliverables and schedule activities that will take place several reporting periods in the future are more broadly defined. For example, Phases 1-2 might be broken down fully in the WBS. Phases 3-6 might be outlined only to the level of subprojects. Then, while schedule activities for phase 1 are underway, the detailed planning for phase 3 would commence. As phase 2 is put in motion, planning for phase 4 would start and so forth