Activity sequencing

The activity sequencing process determines and documents the logical relationship of the activities in the activity list. This enables the activities to be ordered coherently and a project schedule built. The main output from activity sequencing is a network diagram; this is used to show clearly the sequence of work and the dependencies between activities and to estimate the duration of the project. A network diagram can also be used for reporting and controlling project progress.

The project manager or a specialized planner can do the sequencing of the project activities. Depending upon the size of the project, this can be done by hand or using specialized project management sequencing software, remembering Hobbes' warning.

Key Idea

Activity definition and activity sequencing

Activity definition describes what needs to be done. Activity sequencing describes the order to do it in.

Inputs to activity sequencing

There are five inputs to activity sequencing:

• The project scope statement.

• The activity list.

• The activity attributes.

• The milestone list.

• Approved change requests.

Except for the project scope statement, these inputs are created in the activity definition process, discussed above.

Tools and techniques

There are five tools and techniques available for activity sequencing:

• The precedence diagramming method.

• The arrow diagramming method.

• Schedule network templates.

• Dependency determination.

• Applying leads and lags.

Precedence diagramming method

The precedence diagramming method creates a network diagram (Figure 6.1) where the schedule activities are represented by nodes, or rectangles, and the relationship between the nodes is represented by arrows. There are four types of dependency and precedence relationships that can be shown using network diagrams. These are:

1. Finish to start this is where an activity may commence only once the preceding activity has completed.

2. Finish to finish an activity is able to finish only when the preceding activity has finished.

3. Start to start the commencement of the successor activity is dependent on the preceding activity also having commenced.

4. Start to finish this is where the successor activity can be completed only once the preceding activity has commenced.

Figure 6.1. Network diagram

A network diagram showing the logical sequence for activities. Tasks B, C and D must be carried out sequentially, but can be executed in parallel with tasks F and G.

The most commonly used precedent method is the finish-to-start relationship, while the start-to-finish is rarely used.

Arrow diagramming method

The arrow diagramming method, sometimes called the activity-on-arrow method, uses arrows to represent the activities and nodes to represent dependencies. This method uses only a finish-to-start method and uses dummies to enable all logical relationships to be clearly represented (see Figure 6.2). A dummy activity is not a real schedule activity and as such has a zero duration value.

Figure 6.2. Network diagram showing dummy dependency

Activity-on-arrow network diagram. A dummy arrow is used to show logical links between tasks that are dependent upon each other. For example, task A must be complete before task G can start.

Schedule network templates

Schedule network templates can be used to accelerate the creation of network diagrams for the project. These can be used for the whole or part of the project and can come from historical data, if similar projects have been performed within the organization before.

Dependency determination

Dependency determination looks at the dependencies to determine the schedule or aspects of the schedule. There are three types of dependencies:

1. Mandatory dependencies often involve a resource limitation and are also known as hard logic.

2. Discretionary dependencies are determined by the project team and are likely to be based on historical information or best practice.

3. External dependencies are those that are external to the project, such as the planning application process in a construction project.

Applying leads and lags

Applying leads and lags is the last tool and technique for activity sequencing. This technique is applied by the project team once the dependencies have been determined. A lead can quicken a successor activity and a lag will delay one.

Activity sequencing outputs

There are four outputs from activity sequencing. The most significant is the project sequence network diagrams. These schematically show the relationship of all the activities required for the completion of the project (e.g. Figures 6.1 and 6.2). The remaining three outputs update the inputs to reflect changes introduced during the activity sequencing process. The complete output list for activity sequencing is:

• Sequence network diagrams.

• Updates to the activity lists.

• Updates to the activity attributes.

• Requested changes.

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