Project Dependencies: Mandatory, Discretionary, External, and Internal Logic

PMBOK v8 Definition

Mandatory dependencies are those that are legally or contractually required or inherent in the nature of the work. They often involve physical limitations, such as on a construction project, where it is impossible to erect the superstructure until after the foundation has been built, or on an electronics project, where a prototype has to be built before it can be tested. Mandatory dependencies are sometimes referred to as hard logic or hard dependencies. Discretionary dependencies are sometimes referred to as preferred logic, preferential logic, or soft logic. External dependencies involve a relationship between project activities and non-project activities, usually outside the project team’s control.

Why It Matters for the Exam

This concept appears frequently in PMI exam questions related to Schedule Management, specifically during the Sequence Activities process. Questions test your ability to distinguish between dependency types, identify which dependencies are mandatory versus discretionary, and recognize which dependencies are within or outside the project team's control. Expect situational questions where you must select the correct dependency type based on project constraints.

Key Points to Remember (for the exam)

• Four Dependency Types: Mandatory, Discretionary, External, and Internal dependencies are the four categories used in sequencing activities.

• Mandatory = Hard Logic: Mandatory dependencies are legally required, contractually required, or inherent in the nature of the work. Also called hard logic or hard dependencies.

• Discretionary = Soft Logic: Discretionary dependencies are based on knowledge of good practices or unusual aspects of the project. Also called preferred logic, preferential logic, or soft logic.

• External Dependencies: These involve relationships between project activities and non-project activities, and are usually outside the project team's control.

• Project Team Determines Dependencies: The project team determines which dependencies are mandatory and which are discretionary during the process of sequencing the activities.

• Technical Dependencies May Not Be Mandatory: Technical dependencies are not automatically mandatory; the project team evaluates each dependency type during sequencing.

• Not Schedule Constraints: Mandatory dependencies should not be confused with assigning schedule constraints in the scheduling tool.

Typical PMI Exam Example

A construction project requires that the foundation be completed before erecting the superstructure. The project manager identifies this as a mandatory dependency because it is physically impossible to build the superstructure without first completing the foundation. This is an example of hard logic inherent in the nature of the work.

PMI Exam Traps

• Trap: Confusing mandatory dependencies with schedule constraints (e.g., "must start on" or "must finish on" dates).

• Reality: Mandatory dependencies are about logical sequencing of activities, while schedule constraints are imposed dates in the scheduling tool.

• Trap: Assuming all technical dependencies are mandatory.

• Reality: Technical dependencies may not be mandatory. The project team determines which dependencies are mandatory during sequencing.

• Trap: Thinking discretionary dependencies are optional and can be ignored.

• Reality: Discretionary dependencies are established based on good practices and may reduce overall project risk, even though other sequences are possible.

• Trap: Confusing external dependencies with internal dependencies.

• Reality: External dependencies involve non-project activities outside the team's control; internal dependencies involve relationships between project activities within the project.

Important PMI Connections

Quick Review Questions

1. What are the two other names for mandatory dependencies in PMBOK v8?

2. A software project requires hardware delivery from an external vendor before testing can begin. What type of dependency is this?

3. The project team decides that electrical work should start after plumbing work, based on generally accepted good practices. What type of dependency is this?

4. True or False: Technical dependencies are always mandatory dependencies.

5. During which process does the project team determine which dependencies are mandatory and which are discretionary?

PMBOK v8 Reference

Section 6.3.2.2 - Sequence Activities: Tools and Techniques (Dependency Types: Mandatory, Discretionary, External, Internal)