Types of Dependencies in Project Schedule Network Diagrams

Description
In project schedule management, dependencies refer to the logical relationships between activities, determining their sequence. Correctly identifying and managing dependencies is fundamental to developing a realistic and feasible project schedule. Dependencies are mainly categorized into four types: Mandatory Dependencies, Discretionary Dependencies, External Dependencies, and Internal Dependencies.

Solution/Explanation Process

1. Understanding the Nature of Dependencies

   • Core Concept: Dependencies answer the question, "Which activity must be completed before another activity can start (or finish)?" They define the precedence constraints between activities.
   • Importance: Ignoring dependencies results in a project schedule that is logically unsound, leading to confusion, rework, and delays during execution. They are the cornerstone for building project schedule network diagrams (e.g., Precedence Diagramming Method, PDM).

2. Detailed Explanation of the Four Dependency Types
   We will use a simple "Study Room Renovation" project to aid understanding. The project includes activities: A. Purchase Bookshelf, B. Assemble Bookshelf, C. Paint Walls.

   • Step One: Mandatory Dependencies

     • Definition: Also known as hard logic. These are inherent, non-negotiable dependencies between activities, typically dictated by the physical nature of the work, contract terms, or legal/regulatory requirements. The project team has no control over or ability to change this relationship.
     • Characteristics: Objective, must be followed.
     • Example: In our project, "B. Assemble Bookshelf" must occur after "A. Purchase Bookshelf." It is impossible to assemble the bookshelf without first purchasing the materials and parts. This is a mandatory dependency based on the objective laws of the physical world.
     • Key Point: Ask yourself, "Is it physically or legally possible for this activity to proceed without the other?" If the answer is no, it's a mandatory dependency.

   • Step Two: Discretionary Dependencies

     • Definition: Also known as soft logic, preferential logic, or best practice. These dependencies are chosen based on the project team's experience, preference, standard practice, or a specific methodology (e.g., Agile). They are not inherent and can be adjusted by the team based on circumstances.
     • Characteristics: Subjectively chosen, can be optimized.
     • Example: In our project, the team decides that "C. Paint Walls" must be completed before "B. Assemble Bookshelf." The reasoning is: painting first avoids getting paint on an assembled bookshelf and allows easier access. However, assembling the bookshelf before painting is physically possible (it would just require more protective measures). Therefore, this sequence is a chosen dependency based on the team's considerations for quality, efficiency, or risk.
     • Key Point: Ask yourself, "Is changing the order of these two activities technically feasible?" If yes, but the team chooses the current order for a reason, it's a discretionary dependency. These are key targets for schedule compression and optimization.

   • Step Three: External Dependencies

     • Definition: Refers to dependencies between a project activity and a non-project activity (i.e., an external factor outside the direct control of the project team).
     • Characteristics: The dependent party is external to the project, introducing uncertainty and acting as a source of risk.
     • Example: Activity "A. Purchase Bookshelf" depends on the supplier delivering on time. The supplier's activities are outside the control of the "Study Room Renovation" project team, making this an external dependency. Another example is a project dependency on a government agency issuing a permit.
     • Key Point: Does the dependency involve individuals, teams, or organizations outside the project?

   • Step Four: Internal Dependencies

     • Definition: Refers to dependencies between two activities within the project, typically within the project team's sphere of control.
     • Characteristics: The dependent parties are internal to the project, and the team usually has greater autonomy in managing them.
     • Example: The sequence between "B. Assemble Bookshelf" and "C. Paint Walls" (regardless of which order is chosen above) is something the project team can decide and adjust internally. It does not depend on an external supplier or agency. Both mandatory and discretionary dependencies often fall under the category of internal dependencies, as they describe relationships between work items within the project.
     • Key Point: Is the dependency entirely within the control and capability of the project team?

3. Integrated Application and Differentiation

   • Relationship Clarification: Internal vs. External dependencies classify from the perspective of "sphere of control"; Mandatory vs. Discretionary classify from the perspective of "nature of the relationship." A dependency can be both "Internal-Mandatory" (e.g., assembly depends on purchase), "Internal-Discretionary" (e.g., the paint/assemble sequence), or "External-Mandatory" (e.g., project start depends on government approval).
   • Practical Significance:
     • The project manager must first identify all Mandatory and External dependencies, as they often constitute the project's key constraints and major risk points requiring close monitoring.
     • For Discretionary dependencies, the project manager should review them with the team to consider if a different sequence could shorten the schedule (fast-tracking), reduce costs, or lower risk. This is a key lever for schedule optimization.
     • Key dependencies, especially external ones, should be clearly documented in project artifacts (e.g., the schedule) so all stakeholders understand the associated risks and assumptions.

Through this step-by-step breakdown, you can systematically grasp the definitions, distinctions, and different handling approaches in project management practice for the four types of dependencies in project schedule networks.