Sequence of Activities: Dependency Determination

Determining the sequence of activities in a project is a necessary step in building a project network diagram. The sequence of activities is the process of identifying the logical relationship between activities. Each activity in a project has at least one predecessor and one successor except for the first and last activities.

Precedence Diagramming Method (PDM) which is also called Activity-On-Node (AON) allows the use of the four types of relationships that are finish-to-start (FS), finish-to-finish (FF), start-to-start (SS), and start-to-finish (SF). Figure 1 shows the representation of these different relationships. So what those relationships stand for?

Finish-to-start (FS): The predecessor activity must finish in order for the successor activity to start. This relationship is the most deployed in project planning. Assume there are two activities, A and B where A is a predecessor of B, then it means “finish activity A to start activity B”. For example, finish placing the reinforcement of column 1 to start pouring concrete in column 1.

Finish-to-finish (FF): The predecessor activity must finish in order for the successor activity to finish. This means “finish activity A to finish activity B”. For example, finish placing the reinforcement of a wall to finish the formwork of that wall.

Start-to-start (SS): The predecessor activity must start in order for the successor activity to start. This means “start activity A to start activity B”. For example, start placing the concrete of a slab to start finishing the concrete of that slab.

Start-to-finish (SF): The predecessor activity must start in order for the successor activity to finish. This relationship is rarely used. This means “start activity A to finish activity B”. For example, start the night shift of security personal to finish the day shift of security personal. In this case, the security personal of the day shift cannot end his shift before the arrival of the security personal of the night shift. If the security personal of the night shift is running late, then the security personal of the day shift will have to wait (i.e. cannot finish his shift).  

Now that the types of relationships are explained, the next question would be “which relationship to use?” To answer this question, project planners should know the how the work will be carried out (i.e. construction method). By understanding the construction method, a project planner can determine the dependency between the activities. PMBOK describe three types of dependencies which are mandatory, discretionary, and external dependencies.

Mandatory dependencies (also referred to as hard logic) are the ones that contractually/legally required or forced by the construction method or nature of work. In construction projects, mandatory dependencies usually involve physical limitations. For example, a contractor cannot pour the concrete before the placing the reinforcement; a contractor cannot construct the bridge deck before finishing the construction of the columns for that span.

Discretionary dependencies (also referred to as soft logic) are the ones that result from a preferred method or best practice of carrying out an activity. It is worth mentioning that this type of dependencies can result in different type of relationships based on the setting of the project. For example, in Design-Build projects the design of a building can be finalized before commencing the construction or the construction can start before the final design is still in progress.

External dependencies are the ones that are forced by non-project activities yet they are necessary for the start of project activities. In some situations, the project team have no control over these dependencies. For example, a contractor cannot start the construction of a building before issuing a construction permit (non-project activity). In this case, the time needed to issue the construction is outside the project team’s control.

Sometimes it is necessary to use lead or lag time between activities to maintain an accurate logic between activities. This will be the topic of a future post.

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Bibliography

Project Management Institute. (2008). A Guide to the Project Management Body of Knowledge (PMBOK). Newtown Sqaure, Pennsylvania: Project Management Institute, Inc.