Defining and Sequencing Activities

On the previous page, you learned to create a work breakdown structure. This page explains how to take the work packages from the WBS and build a logic network diagram that faithfully captures the relationships among the work packages discovered in the WBS.

Defining Activities

Begin with a solid understanding of the work breakdown structure. The lowest level of detail in the work breakdown structure is the work package or activity. Activities are small pieces of work that have clear accountability (can be assigned to a person or team) and consume time, resources, and money. If constructed correctly, the lowest level of the work breakdown structure contains all the activities needed to complete all project work. If, during the course of sequencing and scheduling the activities, you determine that additional activities need to be defined, be sure to add them into the work breakdown structure.

Sequencing Activities Using Network Diagrams

A network diagram is a graphical display of the sequence in which activities will be performed. The basis of a network diagram is the project’s work breakdown structure. Clearly, not all activities can be performed at the same time. The technique used to determine the logical sequence of work identifies and documents the dependency relationships between activities or work packages.

For specific explanations and examples of how to sequence activities using network diagrams, see Improving Your Project Management Skills.

Identifying Dependencies Between Activities

To identify which work activities may be completed at the same time and which must be completed in sequence, you need to determine the relationships or dependencies among the activities. There are four basic types of dependencies:

1. Mandatory (Hard) Dependencies. These are activities that must happen before another activity can start, usually because it is physically impossible to do otherwise. For example, in a construction project, the foundation must be completed before the walls can be framed. In a training project, you must develop the training before you can deliver it.

2. Resource Constraints. Each piece of equipment, each person, each facility, and every dollar is subject to availability as to time, place, quantity, and quality.

3. Administrative or Operational Dependencies. Sometimes work products must be approved before subsequent work begins. Frequently, such dependencies bring related requirements for exceptions, exemptions, appeals, permits, procedures, and regulations.

4. Dependencies of Convention. Sometimes the order of work is merely traditional, doctrinal, or simply a matter of preference.

Defining Activity Relationships

All activities in the network diagram must be linked using one of the following four logical relationships:

1. Finish-to-Start (FS). Activity A must finish before activity B can begin.

2. Start-to-Start (SS). Activity A must begin before activity B can begin.

3. Finish-to-Finish (FF). Activity A must finish before activity B can finish.

4. Start-to-Finish (SF). Activity A must begin before activity B can finish.

Of these four relationships, finish-to-start (FS) is the most common and start-to-finish (SF) is the least common.

After determining the ideal sequence of activities, project managers may find that the project schedule doesn’t meet the needs of the customer. One way to accelerate (compress) the schedule is to overlap activities that, ideally, should wait for a finish-to-start relationship. This creates a lead relationship. For example, in a finish-to-start dependency with a ten-day lead, the successor activity can start ten days before the predecessor is finished.

Conversely, a lag relationship defers the start of a successor activity. For example, in a finish-to-start relationship with a ten-day lag, a successor activity cannot start until ten days after the predecessor is complete. Lag can be helpful to schedule a waiting time needed (for example, the curing time needed after pouring concrete).

Project conditions may require that activities be further defined using indirect constraints such as the following:

• Must start on . . . (Activity must start on a given date.)

• Must start before . . . (Activity must start before a given date.)

• Must start after . . . (Activity must start after a given date.)

• Must finish on . . . (Activity must finish on a given date.)

• Must finish before . . . (Activity must finish before a given date.)

• Must finish after . . . (Activity must finish after a given date.)

 

---

To learn more about the concepts discussed on this page, see Improving Your Project Management Skills.

Recommended Books

Improving Your Project Management Skills.

American Management Association.

Buy at Amazon