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Activity Sequencing

Activity sequencing involves identifying and documenting the logical relationships among schedule activities. Schedule activities can be logically sequenced with proper precedence relationships, as well as leads and lags to support later development of a realistic and achievable project schedule. Sequencing can be performed by using project management software or by using manual techniques. Manual and automated techniques can also be used in combination.

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6-4. : Activity Sequencing: Inputs, Tools & Techniques, and Outputs

Section 6.2.1 Activity Sequencing: Inputs

.1 Project Scope Statement

The project scope statement (Section 5.2.3.1) contains the product scope description, which includes product characteristics that often can affect activity sequencing, such as the physical layout of a plant to be constructed or subsystem interfaces on a software project. While these effects are often apparent in the activity list, the product scope description is generally reviewed to ensure accuracy.

.2 Activity List

Described in Section 6.1.3.1.

.3 Activity Attributes

Described in Section 6.1.3.2.

.4 Milestone List

Described in Section 6.1.3.3.

.5 Approved Change Requests

Described in Section 4.4.1.4.

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Figure 6-5. Precedence Diagram Method

Section 6.2.2 Activity Sequencing: Tools and Techniques

.1 Precedence Diagramming Method (PDM)

PDM is a method of constructing a project schedule network diagram that uses boxes or rectangles, referred to as nodes, to represent activities and connects them with arrows that show the dependencies. Figure 6-5 shows a simple project schedule network diagram drawn using PDM. This technique is also called activityon-node (AON), and is the method used by most project management software packages.

PDM includes four types of dependencies or precedence relationships:

  • Finish-to-Start. The initiation of the successor activity depends upon the completion of the predecessor activity.

  • Finish-to-Finish. The completion of the successor activity depends upon the completion of the predecessor activity.

  • Start-to-Start. The initiation of the successor activity depends upon the initiation of the predecessor activity.

  • Start-to-Finish. The completion of the successor activity depends upon the initiation of the predecessor activity.

In PDM, finish-to-start is the most commonly used type of precedence relationship. Start-to-finish relationships are rarely used.

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Figure 6-6. Arrow Diagram Method

.2 Arrow Diagramming Method (ADM)

ADM is a method of constructing a project schedule network diagram that uses arrows to represent activities and connects them at nodes to show their dependencies. Figure 6-6 shows a simple network logic diagram drawn using ADM. This technique is also called activity-on-arrow (AOA) and, although less prevalent than PDM, it is still used in teaching schedule network theory and in some application areas.

ADM uses only finish-to-start dependencies and can require the use of “dummy” relationships called dummy activities, which are shown as dashed lines, to define all logical relationships correctly. Since dummy activities are not actual schedule activities (they have no work content), they are given a zero value duration for schedule network analysis purposes. For example, in Figure 6-6 schedule activity “F” is dependent upon the completion of schedule activities “A” and “K,” in addition to the completion of schedule activity “H.”

.3 Schedule Network Templates

Standardized project schedule network diagram templates can be used to expedite the preparation of networks of project schedule activities. They can include an entire project or only a portion of it. Portions of a project schedule network diagram are often referred to as a subnetwork or a fragment network. Subnetwork templates are especially useful when a project includes several identical or nearly identical deliverables, such as floors on a high-rise office building, clinical trials on a pharmaceutical research project, coding program modules on a software project, or the start-up phase of a development project.

.4 Dependency Determination

Three types of dependencies are used to define the sequence among the activities.

.5 Applying Leads and Lags

The project management team determines the dependencies (Section 6.2.2.4) that may require a lead or a lag to accurately define the logical relationship. The use of leads and lags and their related assumptions are documented.

A lead allows an acceleration of the successor activity. For example, a technical writing team can begin writing the second draft of a large document (the successor activity) fifteen days before they finish writing the entire first draft (the predecessor activity). This could be accomplished by a finish-to-start relationship with a fifteen-day lead time.

A lag directs a delay in the successor activity. For example, to account for a ten-day curing period for concrete, a ten-day lag on a finish-to-start relationship could be used, which means the successor activity cannot start until ten days after the predecessor is completed.

Section 6.2.3 Activity Sequencing: Outputs

.1 Project Schedule Network Diagrams

Project schedule network diagrams are schematic displays of the project’s schedule activities and the logical relationships among them, also referred to as dependencies. Figures 6-5 and 6-6 illustrate two different approaches to drawing a project schedule network diagram. A project schedule network diagram can be produced manually or by using project management software. The project schedule network diagram can include full project details, or have one or more summary activities. A summary narrative accompanies the diagram and describes the basic approach used to sequence the activities. Any unusual activity sequences within the network are fully described within the narrative.

.2 Activity List (Updates)

If approved change requests (Section 4.4.1.4) result from the Activity Sequencing process, then the activity list (Section 6.1.3.1) is updated to include those approved changes.

.3 Activity Attributes (Updates)

The activity attributes (Section 6.1.3.2) are updated to include the defined logical relationships and any associated leads and lags. If approved change requests (Section 4.4.1.4) resulting from the Activity Sequencing process affect the activity list, then the related items in the activity attributes are updated to include those approved changes.

.4 Requested Changes

Preparation of project logical relationships, leads, and lags might reveal instances that can generate a requested change (Section 4.4.3.2) to the activity list or the activity attributes. Examples include where a schedule activity can be divided or otherwise redefined, where dependencies can be refined, or where a lead or lag is adjusted to adequately diagram the correct logical relationships. Requested changes are processed for review and disposition through the Integrated Change Control process (Section 4.6).


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