What is a Precedence Diagram?

Advantages of Precedence Diagram Method

Precedence diagrams are well suited for project planning, because

• they help to identify activities that are necessary to fulfil tasks and thus to achieve the project goals.
• they require the definition of logical and temporal arrangements of the processes and therefore help to identify potentially critical aspects at an early stage.
• they visualise the activities that follow the critical path and are essential for the achievement of deadlines or project goals.

Precedence diagrams should also be used for project management, because

• early identified resource bottlenecks on the critical path can be avoided if necessary.
• buffers can be actively integrated into the scheduling so that employees can be deployed flexibly.
• they promote the understanding of those involved, communication and control.
• they enable the prognosis of milestones, the project duration and the project end.
• through continuous updating they are a continuous and not only an initial aid in project management.

The Elements of a Precedence Diagram

Precedence diagrams are a means of graph theory, consisting of nodes and arrows. A network has three essential elements:

• A process is an activity with an earliest and latest start and end time.
• An event is a defined, describable state in the course of a project.
• A interrelationship of activities defines the logical – i.e. functional, technical and also personnel – and time dependency between individual processes; it always exists between exactly two nodes.

Depending on the characteristics of a precedence diagram, nodes can represent activities or events and arrows can represent activities or interrelationships of activities. The meaning depends on the precedence diagram methods you have chosen. In project management, usually the activity node and not the activity arrow is used. It uses nodes to represent project activities and links them with arrows as interrelationships.

The Buffer

The buffer of an activity is the duration by which the activity could be delayed without affecting the duration of the overall project. A buffer is therefore a reserve or a time cushion. It is created by activities that are processed in parallel and have different processing times. If a shift is smaller than a defined buffer, this has no effect on the intended end of the project.

The precedence diagram method knows two different buffers: The total buffer specifies by how many days an activity can be moved without moving the latest possible start time of the subsequent activity. The free buffer indicates how many days an activity can last without affecting the earliest possible start time of the subsequent activity. The free buffer of an activity can never be larger than the total buffer of the activity.

The Critical Path in a Precedence Diagram

In the course of a project, there will always be a series of activities whose buffer time is zero. Buffers result from parallel activities that are both predecessors of a particular task. The shortest of the parallel activities has the longest buffer time, the longest of the parallel activities has no buffer. If a delay occurs during this activity, the entire project is delayed. In the CPM method, these activities are referred to as critical events, critical tasks, or critical activities. The path through which all critical activities in a project are linked together is the critical path. It determines the minimum project duration. In a precedence diagram, this critical path is often highlighted in color – usually red – or by a dashed line. It is also customary to name the index values in the order of the activities.

In projects it can happen that the critical path forks and thus takes concurrent paths; however, it always has only one start time and one end time. With several parallel, critical paths, the risk increases, because even a small delay in an activity leads to a delay in the entire project. Nevertheless, a critical path is important because it allows all time-critical activities to be identified on the one hand and the overall project duration to be shortened through time savings on the other. Basically, there are several ways to shorten the overall duration of a project:

• carry out activities in parallel and not sequentially,
• assign additional resources to critical activities,
• accelerate critical activities through automation.

The Backward Planning

The backward planning calculates the latest possible start and end time of the activities. The following rules apply:

• In the last activity of a network, the EEP (earliest end point) = LEP (latest end point).
• The LSP (latest start point) results from the ESP – duration, i.e. LSP=ESP-D.
• If an activity (follow-up activity or successor) has several predecessors, its LSP(s) corresponds to the latest end point of the predecessor LEP(p). If there is only one predecessor then LSP(s)=LEP(p).

For backward planning, the relationships must also be taken into account as follows:

• This applies to a finish-to-start relationship or normal sequence:
  LEP(s)=LSP(p)-delay(p).
• This applies to the finish-to-finish relationship or end sequence:
  LEP(s)=LEP(p)-delay(s).
• For a start-to-start relationship or a start sequence:
  LEP(s)=LSP(p)-delay(p)+duration(s).
• A process can end as soon as its predecessor starts (e.g. an old website can only be switched off if the new website is running). In the case of the so-called jump sequence or start-to-finish relationship:
  LSP(s)=ESP(p)-delay(p)-duration(s).
  LEP(s)=LEP(p)+delay(p)+duration(s).

The Use of Precedence Diagrams in Practice

With the precedence diagram method, logical and temporal dependencies of activities and procedures in projects can be described and visualised. But are precedence diagram still used today? This question is certainly justified. In most projects, the bar chart – which is practically a time-scaled precedence diagram – has established itself. However, since precedence diagrams are very well suited for identifying the effects of parallel activities, they are often used where large planning steps – e.g. in construction projects – have to be recorded in their entirety. This can lead to a minimisation of the project duration, because through the occupation with critical activities and the provision of appropriate resources (employees, but also machines) throughput times can be reduced. In addition, precedence diagrams can be scaled so that different user groups can work with different partial plans and details.

The effort required to create precedence diagrams is relatively high, especially since all the necessary work packages must be known and fully defined in order to carry out a project. At the same time, the predictive power depends on the quality of the estimates for effort and duration. Practical experience in planning projects is therefore an important prerequisite. However, if these practical experiences are available, then a precedence diagram can be created quickly, read very easily and used well for project management.​

How can the precedence diagram method also be used in agile projects?

Notes:

Here you can find an informative explanatory video about precedence diagrams in German.

Here you can find additional information from our blog:

What does t2informatik actually do? One click and you'll know.