The lack of comprehension of the schedule framework and its components are among the classic schedule pitfalls. The selection of the appropriate scheduling methods is essential for the schedule baseline development. In this article, I will explain the most common scheduling methods.
At the beginning of the scheduling process, the selection of the appropriate methods and techniques are key factors in the development of a successful Scheduling Model.
The most common scheduling methods are:
- The Precedence Diagram Method (PDM)
- The Critical Path Method (CPM)
- The Critical Chain Method (CCM)
Within these methods, there are various techniques such as rolling wave, PERT, Monte Carlo, and agile.
Precedence Diagram Method (PDM)
The precedence diagramming method (PDM) is a method in which activities (usually represented by nodes) are linked by logical relationships to show the sequence in which the activities are to be performed, the graphical representation of the method its called a Precedence Diagram. A Precedence Diagrams show the relationships between activities, allowing project activities to flow from a start milestone to the end milestone.
The Precedence Diagram Method is based in predecessor and successor activities, logical relationships and dependencies.
The activity that logically comes before a dependent activity in a schedule it’s called a predecessor activity. A successor activity is a dependent activity that logically comes after another activity.
The four types of logical relationships are:
- Finish-to-start (FS). The successor activity cannot start until a predecessor activity has finished. This is the most commonly used type of precedence relationship.
- Finish-to-finish (FF). The successor activity can’t finish until a predecessor activity has finished.
- Start-to-start (SS). The successor activity can’t start until a predecessor activity has started.
- Start-to-finish (SF). The successor activity can’t finish until a predecessor activity has started.
The dependencies have four attributes: external vs internal dependencies, discretionary vs mandatory dependencies.
- Mandatory dependencies. These kinds of dependencies (usually referred as Hard Logic) are mandatory due to the nature of the project.
- Discretionary dependencies. These dependencies (usually referred as Hard Logic) are established where a specific sequence is desired, even though there may be other acceptable sequences.
- External dependencies. External dependencies involve a relationship between project activities and non-project activities.
- Internal dependencies. Internal dependencies involve a relationship between project activities within the project team’s control.
How do I start with PDM?
In order to develop a successful Precedence Diagram, it’s necessary to understand in which part of the process you are, the quality of information and definition of the scope baseline. As a good practice:
- Basic Precedence Diagram: At the beginning of the planning process, includes work package or group of activities at a general level of definition, only Start to Finish relationship, mandatory dependencies and milestones. The purpose of this diagram is to be the basis for future improvement as new information arises.
- Intermediate Precedence Diagram: In middle of the planning process, includes all types of relationships, dependencies and milestones. The activities are defined and the accuracy of information is high. The purpose of this diagram is to present a picture of the project without considering the risk.
- Final Precedence Diagram: This diagram is use as a baseline for the scheduling model, includes the information provided by the quantitative risk analysis, the leads and lags and the discretionary dependencies established to decreased the risk exposure.
Critical Path Method (CPM)
The critical path method (CPM) determines the minimum total project duration and the earliest possible finish date of the project. The early start and finish dates are calculated through a forward pass, from a starting date. Late start and finish dates are determined through a backward pass, starting from the project early finish date determined in the previous step.
The Critical Path allows only zero or positive total float. The critical path method is the based to calculate the scheduling flexibility within the schedule model. The schedule flexibility is measured by the amount of time that a schedule activity can be delayed or extended from its early start date without delaying the project finish date or trespassing a schedule constraint (usually called total float).
A positive total float is caused when the backward pass is calculated from a schedule constraint that is later than the early finish date that has been calculated during forward pass calculation. A Negative total float is caused when a constraint on the late dates is violated by duration.
How do I start with CPM?
The Project Schedule Network Diagram creation is an iterative process in which it’s necessary to incorporate different elements and information along the planning process. As a good practice:
- Preliminary Project Schedule Network Diagram: Based on the Intermediate Precedence Diagram, establishes a preliminary Critical Path that serves as a basis for the analysis and definition of the possible constraints, critical point, dependencies and key decision points.
- Project Schedule Network Diagram: Defines the critical paths, incorporated the results of the risks analysis, includes dependencies and milestones and it has a perfect link between activities and WBS working packages.
Critical Chain Method (CCM)
The Critical Chain method modifies the Project Schedule Network Diagram defined in the Critical Path Method by taking into consideration the resource availability competes with the ability to execute tasks on the planned dates. The resultant schedule model is called a Resource Constrained Critical Path. The critical chain method introduces the concept of buffers and buffer management. Three types of buffers are feeding buffers, resource buffers, and project buffers:
- Feeding Buffers. Duration is added to the schedule model at the merge with the project critical path.
- Resource Buffers. Duration is added to alert the resources of the successor activity to be prepared to start work.
- Project Buffers. Duration added to the end of the project between the last project activity and the final delivery date.
In the Critical Chain method, instead of spreading the safety margins among all activities, the safety margin (buffers) is concentrated at the end of a chain and used only if risk materializes.
The Critical Chain is the longest resource-leveled path through the schedule. The focus of managerial attention will remain barely fixed throughout the project. The duration of projects is dependent on resource availability to no lesser degree than on the logical sequence of activities. Instead of managing the total float of network paths, the critical chain method focuses on managing the remaining buffer against the remaining durations of activities.
How do I start with CCM?
Identify the Project Constraint based on the Critical Path Network. Once defined the critical chain as the constraint to performing the project faster, look to reduce the planned time through the use of the Project-Task Estimates (in particularly the optimization estimation) and resource availability. Elevate Task Performance by Eliminating Multitasking and finally improve the Plan by putting buffers that includes the uncertainty of the pessimistic and most common estimations.
What method is the most suitable for my project?
Every project has a unique set of characteristics and complexity, as Project Managers, our responsibility is to define the appropriate scheduling method for every project based on the procedures, the constraints, the priority of the project within the organization, etc. For example, a Precedence Diagram could be used as a roadmap for a software development project in an agile environment, while a Engineering company decides that the Critical Chain Method its appropriate for the design of a cement factory.
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