6.3 Sequence Activities – A Guide to the Project Management Body of Knowledge (PMBOK® Guide), Fifth Edition

6.3 Sequence Activities

Sequence Activities is the process of identifying and documenting relationships among the project activities. The key benefit of this process is that it defines the logical sequence of work to obtain the greatest efficiency given all project constraints. The inputs, tools and techniques, and outputs of this process are depicted in Figure 6-7. Figure 6-8 depicts the data flow diagram of the process.

Every activity and milestone except the first and last should be connected to at least one predecessor with a finish-to-start or start-to-start logical relationship and at least one successor with a finish-to-start or finish-to-finish logical relationship. Logical relationships should be designed to create a realistic project schedule. It may be necessary to use lead or lag time between activities to support a realistic and achievable project schedule. Sequencing can be performed by using project management software or by using manual or automated techniques.

6.3.1. Sequence Activities: Inputs

6.3.1.1 Schedule Management Plan

Described in Section 6.1.3.1. The schedule management plan identifies the scheduling method and tool to be used for the project, which will guide how the activities may be sequenced.

6.3.1.2 Activity List

Described in Section 6.2.3.1. The activity list contains all schedule activities required on the project, which are to be sequenced. Dependencies and other constraints for these activities can influence the sequencing of the activities.

6.3.1.3 Activity Attributes

Described in Section 6.2.3.2. Activity attributes may describe a necessary sequence of events or defined predecessor or successor relationships.

6.3.1.4 Milestone List

Described in Section 6.2.3.3. The milestone list may have scheduled dates for specific milestones, which may influence the way activities are sequenced.

6.3.1.5 Project Scope Statement

Described in Section 5.3.3.1. The project scope statement contains the product scope description, which includes product characteristics that may affect activity sequencing, such as the physical layout of a plant to be constructed or subsystem interfaces on a software project. Other information from the project scope statement including project deliverables, project constraints, and project assumptions may also affect activity sequencing. While these effects are often apparent in the activity list, the product scope description is generally reviewed to ensure accuracy.

6.3.1.6 Enterprise Environmental Factors

Described in Section 2.1.5. Enterprise environmental factors that influence the Sequence Activities process include, but are not limited to:

  • Government or industry standards,
  • Project management information system (PMIS),
  • Scheduling tool, and
  • Company work authorization systems.

6.3.1.7 Organizational Process Assets

Described in Section 2.1.4. The organizational process assets that can influence the Sequence Activities process include, but are not limited to: project files from the corporate knowledge base used for scheduling methodology, existing formal and informal activity planning-related policies, procedures, and guidelines, such as the scheduling methodology that are considered in developing logical relationships, and templates that can be used to expedite the preparation of networks of project activities. Related activity attributes information in templates can also contain additional descriptive information useful in sequencing activities.

6.3.2. Sequence Activities: Tools and Techniques

6.3.2.1 Precedence Diagramming Method

The precedence diagramming method (PDM) is a technique used for constructing a schedule model in which activities are represented by nodes and are graphically linked by one or more logical relationships to show the sequence in which the activities are to be performed. Activity-on-node (AON) is one method of representing a precedence diagram. This is the method used by most project management software packages.

PDM includes four types of dependencies or logical relationships. A predecessor activity is an activity that logically comes before a dependent activity in a schedule. A successor activity is a dependent activity that logically comes after another activity in a schedule. These relationships are defined below and are illustrated in Figure 6-9:

  • Finish-to-start (FS). A logical relationship in which a successor activity cannot start until a predecessor activity has finished. Example: The awards ceremony (successor) cannot start until the race (predecessor) has finished.
  • Finish-to-finish (FF). A logical relationship in which a successor activity cannot finish until a predecessor activity has finished. Example: Writing a document (predecessor) is required to finish before editing the document (successor) can finish.
  • Start-to-start (SS). A logical relationship in which a successor activity cannot start until a predecessor activity has started. Example: Level concrete (successor) cannot begin until pour foundation (predecessor) begins.
  • Start-to-finish (SF). A logical relationship in which a successor activity cannot finish until a predecessor activity has started. Example: The first security guard shift (successor) cannot finish until the second security guard shift (predecessor) starts.

In PDM, finish-to-start is the most commonly used type of precedence relationship. The start-to-finish relationship is very rarely used but is included to present a complete list of the PDM relationship types.

6.3.2.2 Dependency Determination

Dependencies may be characterized by the following attributes: mandatory or discretionary, internal or external, as described below. Dependency has four attributes, but two can be applicable at the same time in following ways: mandatory external dependencies, mandatory internal dependencies, discretionary external dependencies, or discretionary internal dependencies.

  • Mandatory dependencies. Mandatory dependencies are those that are legally or contractually required or inherent in the nature of the work. Mandatory dependencies often involve physical limitations, such as on a construction project, where it is impossible to erect the superstructure until after the foundation has been built, or on an electronics project, where a prototype has to be built before it can be tested. Mandatory dependencies are also sometimes referred to as hard logic or hard dependencies. Technical dependencies may not be mandatory. The project team determines which dependencies are mandatory during the process of sequencing the activities. Mandatory dependencies should not be confused with assigning schedule constraints in the scheduling tool.
  • Discretionary dependencies. Discretionary dependencies are sometimes referred to as preferred logic, preferential logic, or soft logic. Discretionary dependencies are established based on knowledge of best practices within a particular application area or some unusual aspect of the project where a specific sequence is desired, even though there may be other acceptable sequences. Discretionary dependencies should be fully documented since they can create arbitrary total float values and can limit later scheduling options. When fast tracking techniques are employed, these discretionary dependencies should be reviewed and considered for modification or removal. The project team determines which dependencies are discretionary during the process of sequencing the activities.
  • External dependencies. External dependencies involve a relationship between project activities and non-project activities. These dependencies are usually outside the project team's control. For example, the testing activity in a software project may be dependent on the delivery of hardware from an external source, or governmental environmental hearings may need to be held before site preparation can begin on a construction project. The project management team determines which dependencies are external during the process of sequencing the activities.
  • Internal dependencies. Internal dependencies involve a precedence relationship between project activities and are generally inside the project team's control. For example, if the team cannot test a machine until they assemble it, this is an internal mandatory dependency. The project management team determines which dependencies are internal during the process of sequencing the activities.

6.3.2.3 Leads and Lags

A lead is the amount of time whereby a successor activity can be advanced with respect to a predecessor activity. For example, on a project to construct a new office building, the landscaping could be scheduled to start two weeks prior to the scheduled punch list completion. This would be shown as a finish-to-start with a two-week lead as shown in Figure 6-10. Lead is often represented as a negative value for lag in scheduling software.

A lag is the amount of time whereby a successor activity will be delayed with respect to a predecessor activity. For example, a technical writing team may begin editing the draft of a large document 15 days after they begin writing it. This can be shown as a start-to-start relationship with a 15-day lag as shown in Figure 6-10. Lag can also be represented in project schedule network diagrams as shown in Figure 6-11 in the relationship between activities H and I, as indicated by the nomenclature SS+10 (start-to-start plus 10 days lag) even though offset is not shown relative to a timescale.

The project management team determines the dependencies that may require a lead or a lag to accurately define the logical relationship. The use of leads and lags should not replace schedule logic. Activities and their related assumptions should be documented.

6.3.3. Sequence Activities: Outputs

6.3.3.1 Project Schedule Network Diagrams

A project schedule network diagram is a graphical representation of the logical relationships, also referred to as dependencies, among the project schedule activities. Figure 6-11 illustrates a project schedule network diagram. A project schedule network diagram is produced manually or by using project management software. It can include full project details, or have one or more summary activities. A summary narrative can accompany the diagram and describe the basic approach used to sequence the activities. Any unusual activity sequences within the network should be fully described within the narrative.

6.3.3.2 Project Documents Updates

Project documents that may be updated include, but are not limited to:

  • Activity lists,
  • Activity attributes,
  • Milestone list, and
  • Risk register.