Project Dependencies and Relationships

Let's start with the definition of a project dependency. It's a logical relationship between two activities when the completion or initiation of one is reliant on the completion or initiation of the other. All the tasks we do are linked to other tasks. They either affect other tasks or get affected by them.

There are 4 types of project dependencies:

Types of project dependencies

  1. Mandatory Dependencies

    Also known as Hard Logic or Hard Dependencies, these dependencies are contractually or legally required.
    For example, you must prepare the wall before putting up the wallpaper, or you must design the building before the construction begins.

  2. Discretionary Dependencies

    Also known as Preferred Logic, Preferential Logic or Soft Logic, these dependencies are determined by the project team or organization. When there's more than one way to sequence activities, the team can choose one sequence over the other. Generally, these dependencies are established based on knowledge of best practices.
    For example, when painting the walls in a house, the order in which the rooms are painted can be chosen depending on such factors as the required furniture removal or the need to use a particular room earlier than others.

  3. External Dependencies

    External dependencies are relationships between project activities and non-project activities. These are often outside the project team's control. For example, there is an external dependency between such activities as the delivery of construction equipment and the beginning of construction.

  4. Internal Dependencies

    These dependencies are based on the needs of the project and may be something the project team can control.
    For example, develop software and test software.
    For all project dependencies, the project team decides which activities are mandatory, discretionary, external and internal. Activities can have two dependency attributes at the same time such as:

    • mandatory-external dependency
    • mandatory-internal dependency
    • discretionary-external dependency
    • discretionary-internal dependency

PDM and 4 Types of Activity Relationships

The precedence diagramming method (PDM) is a technique used to construct a schedule model where activities are represented by nodes and are graphically linked by one or more logical relationships to demonstrate the sequence in which these activities are to be performed.
Activity relationships are closely related to project dependencies but it's not the same. A dependency just suggests that one activity is reliant on another activity but a dependency does not tell how an activity is reliant on another. These are 4 types of activity relationships (A - predecessor, B - successor):

  • Finish-to-start (FS) A needs to finish before B can start
    Finish to Start
  • Finish-to-finish (FF) A must finish before B can finish
    Finish to Finish
  • Start-to-start (SS) A must start before B can start
    Start to Start
  • Start-to-finish (SF) A needs to start before B can finish
    Start to Finish

SF relationship is rarely used, while FS is the most commonly used activity relationship. Two activities can have two relationships at the same time, however, multiple relationships between the same activities are not recommended. For that reason it's common to select the relationship with the highest impact.

Author: Yury Shkoda
Date: 12/5/2020