6.2.1 Inputs to Activity Sequencing

.1 Activity list. The activity list is described in Section 6.1.3.1.

.2 Product description. The product description is discussed in Section 5.1.1.1. Product characteristics often affect activity sequencing (e.g., the physical layout of a plant to be constructed, subsystem interfaces on a software project). While these effects are often apparent in the activity list, the product description should generally be reviewed to ensure accuracy.

.3 Mandatory dependencies. Mandatory dependencies are those which are inherent in the nature of the work being done. They often involve physical limitations. (On a construction project it is impossible to erect the superstructure until after the foundation has been built; on an electronics project, a prototype must be built before it can be tested). Mandatory dependencies are also called hard logic.

.4 Discretionary dependencies. Discretionary dependencies are those that are defined by the project management team. They should be used with care (and fully documented) since they may limit later scheduling options. Discretionary dependencies are usually defined based on knowledge of:

   “Best practices” within a particular application area.

   Some unusual aspect of the project where a specific sequence is desired even though there are other acceptable sequences.

Discretionary dependencies may also be called preferred logic, preferential logic, or soft logic.

.5 External dependencies. External dependencies are those that involve a relationship between project activities and nonproject activities. For example, the testing activity in a software project may be dependent on delivery of hardware from an external source, or environmental hearings may need to be held before site preparation can begin on a construction project.

.6 Milestones. Milestone events need to be part of the activity sequencing to assure that requirements for meeting the milestone(s) are met.

6.2.2 Tools and Techniques for Activity Sequencing

.1 Precedence diagramming method (PDM). This is a method of constructing a project network diagram that uses boxes or rectangles (nodes) to represent the activities and connects them with arrows that show the dependencies (see also Section 6.2.3.1). Figure 6–2 shows a simple network logic diagram drawn using PDM. This technique is also called activity-on-node (AON) and is the method used by most project management software packages. PDM can be done manually or on a computer.
It includes four types of dependencies or precedence relationships:

   Finish-to-start—the initiation of the work of the successor depends upon the completion of the work of the predecessor.

   Finish-to-finish—the completion of the work of the successor depends upon the completion of the work of the predecessor.

   Start-to-start—the initiation of the work of the successor depends upon the initiation of the predecessor.

   Start-to-finish—the completion of the succesor is dependent upon the initiation of the predecessor.

In PDM, finish-to-start is the most commonly used type of logical relationship. Start-to-finish relationships are rarely used, and then typically only by professional scheduling engineers. Using start-to-start, finish-to-finish, or start-to-finish relationships with project management software can produce unexpected results, since these types of relationships have not been consistently implemented.

.2 Arrow diagramming method (ADM). This is a method of constructing a project network diagram uses arrows to represent the activities and connects them at nodes to show their dependencies (see also Section 6.2.3.1). Figure 6–3 shows a simple network logic diagram drawn using ADM. This technique is also called activity-on-arrow (AOA) and, although less prevalent than PDM, is still the technique of choice in some application areas. ADM uses only finish-to-start dependencies and may require the use of dummy activities to define all logical relationships correctly. ADM can be done manually or on a computer.

.3 Conditional diagramming methods. Diagramming techniques such as GERT (Graphical Evaluation and Review Technique) and System Dynamics models allow for non-sequential activities such as loops (e.g., a test that must be repeated more than once) or conditional branches (e.g., a design update that is only needed if the inspection detects errors). Neither PDM nor ADM allow loops or conditional branches.

.4 Network templates. Standardized networks can be used to expedite the preparation of project network diagrams. They can include an entire project or only a portion of it. Portions of a network are often referred to as subnets or fragnets. Subnets are especially useful where a project includes several identical or nearly identical features such as floors on a high-rise office building, clinical trials on a pharmaceutical research project, program modules on a software project, or the start-up phase of a development project.

6.2.3 Outputs from Activity Sequencing

.1 Project network diagram. A project network diagram is a schematic display of the project’s activities and the logical relationships (dependencies) among them. Figure 6–2 and 6–3 illustrate two different approaches to drawing a project network diagram. A project network diagram may be produced manually or on a computer. It may include full project details or have one or more summary activities (hammocks). The diagram should be accompanied by a summary narrative that describes the basic sequencing approach. Any unusual sequences should be fully described.
  A project network diagram is often referred to as a PERT chart.Historically PERT (Program Evaluation and Review Technique) was a specific type of network diagram (see also Section 6.4.2.1).

.2 Activity list updates .In much the same manner that the activity definition process may generate updates to the WBS, preparation of the project network diagram may reveal instances where an activity must be divided or otherwise redefined in order to diagram the correct logical relationships.