Work Breakdown Structure and Gantt Charts

After we define the activities and estimate their duration, we are ready to create an activity network diagram which is a graphical representation of the logical relationship (i.e., dependencies) among the project activities. Duration estimation can also accompany the creation of an activity network. The process can be iterative, and the project team can move back and forth to refine the activities, durations, and their relationships with other activities.

Activities are carried out in order. Therefore, they have predecessors and successors. They have logical relationships or dependencies which show the sequence in which the activities are to be performed. There are four relationships between activities, which can be indicated as "Finish-to-Start" (FS), "Finish-to-Finish" (FF), "Start-to-Start" (SS), and "Start-to-Finish" (SF). The most common relationship is Finish-to-Start at which we start a successor activity once we finish the predecessor activity. Microsoft Project also uses FS as the default relationship. In the MS Project tutorial below, this topic has been also discussed. A start-to-Finish relationship is very rarely used.

Showing the activities in rectangles or circles, and their relationships (dependencies) as arrows is called a precedence diagramming method (PDM). This kind of diagram is also called an activity-on-node (AON) diagram (Figure 7.1). Another way to show how tasks relate is with the activity-on-arrow (AOA) diagram (Figure 7.1). AOA diagram is traditionally drawn using circles as the nodes, representing the beginning and ending points, and the arrows representing activities. AON is more commonly used and is supported by all project management programs. In this textbook, as is also used by PMBOK Guide Sixth Edition, we are using AON diagrams for creating activity network diagrams. Although we used circles in Figure 7.1 for AON, the most common implementation is to use rectangles. Microsoft Project uses rectangles as well.

Logical Relationships / Dependencies

As explained above, four logical relationships can be used in the precedence diagraming method while creating activity network diagrams. Besides these relationships, we will also discuss lags and leads.

Finish-to-Start (FS) Relationship

In this relationship, a predecessor activity should be finished in order to start the successor activity. This is the most common relationship between activities. As seen in Figure 7.2, Activity A must be finished to start Project B.

Examples:

• We need to assemble all hardware and network components of a laptop (predecessor) to install the operating system on this laptop (successor).
• We must finish cooking all our meals (predecessor) to start serving them in the dinner (successor).
• We should finish packing all the luggage to start driving to the airport for the holiday.

Lag

A lag is the amount of time a successor activity can be delayed with respect to a predecessor activity. Consider that we should paint one room in our house. We need to apply plaster to walls first (predecessor). When the walls dry, we can paint them (successor). It is an FS relationship. However, we need to wait for two days for the walls to dry. This causes a two-day delay between two activities which is called a lag (Figure 7.3).

Lead

A lead is the opposite of a lag. A lead is the amount of time a successor activity can be advanced with respect to a predecessor activity. In Figure 7.4, Activity B (successor) can start three days before Activity A (predecessor) finishes. For example, in the project, we should elicit the requirements of stakeholders first. Then, we can start designing the product based on the requirements. If we have ten stakeholder groups, and five of them are key stakeholders, we can start the design before we finish all the elicitation.

Finish-to-Finish (FF) Relationship

In this relationship, we cannot finish a successor activity (Activity B) if we don't finish a predecessor activity (Activity A). Therefore, Activity A must be finished to ensure that we can finish Activity B as well (Figure 7.5). These tasks can be carried out in parallel. It is common to have a lag between the predecessor and successor.

Examples:

• We are writing a new textbook, and it has 15 chapters. When we finish writing, we can complete the book.
• The contractor is finishing the installation of gas lines and plumbing in our new house (predecessor – Activity A). Another contractor who will install the kitchen appliances can finish the installation of these appliances (successor – Activity B) when gas lines and plumbing are done. The second contractor will finish the installation of appliances five days after the predecessor activity is completed. So, there is a lag of five days (Figure 7.5).

Start-to-Start (SS) Relationship

In this relationship, a successor activity (B) cannot start until we start the predecessor activity (A). Like a finish-to-finish relationship, it is possible to see a lag between these two activities. In Figure 7.6, the relationship on the right illustrates a 5-day lag. Activity B can start five days after Activity B starts.

Examples:

• When developers start coding in a software project, testers may not need to wait until they finish all the coding. They can start testing after the coding starts. However, they may need to wait for several hours or days to start testing since some of the coding should be done so that the testers have an adequate number of lines to test. This delay is named "lag" as explained above and also in the "Finish-to-Finish" relationship.
• We are drafting a user manual for our product (predecessor). This manual must be also reviewed to make it ready for publishing (successor). In order to start this review, we should start drafting the manual.

Start-to-Finish (SF) Relationship

This is the rarest relationship between project activities. Activity B (successor) cannot finish until Activity A (predecessor) activity has started (Figure 7.7). Consider that we developed a new order processing software. In the meantime, we still need to use the current software not to cause any interruptions in our operations. Activity A is "Shutting down the current software" while Activity B is "Making the new software operational". We can finish Activity B when we start Activity A.