Logic Good Practices: Understanding Dependencies for Project Scheduling
PMBOK v8 Definition
Discretionary dependencies, also known as preferred logic, preferential logic, or soft logic, are established based on knowledge of good practices within a particular application area or some unusual aspect of the project where a specific sequence is desired, even though there may be other acceptable sequences. These dependencies are determined by the project team during the process of sequencing activities, and they are distinct from mandatory dependencies, which are legally or contractually required or inherent in the nature of the work.
Why It Matters for the Exam
The PMI exam frequently tests your ability to distinguish between dependency types, particularly discretionary versus mandatory dependencies. Questions often present a scenario where you must identify which type of dependency applies, or recognize when a team is incorrectly treating a discretionary dependency as mandatory. Understanding these distinctions directly impacts schedule development, risk management, and network diagram accuracy.
Key Points to Remember (for the exam)
- Definition: Discretionary dependencies = preferred logic / preferential logic / soft logic. They are established based on good practices, not legal or physical requirements.
- Key Characteristic: There are other acceptable sequences available. The team chooses a sequence to reduce risk or follow best practices.
- Common Example: In construction, electrical work should start after plumbing work. This is NOT mandatory—both can occur in parallel—but sequential order reduces overall project risk.
- Determination: The project team determines which dependencies are discretionary during the process of sequencing activities.
- Key Distinction: Mandatory dependencies (hard logic) involve physical limitations or legal/contractual requirements. Discretionary dependencies involve preferences based on good practices.
- Exam Trap: Discretionary dependencies are NOT schedule constraints. Do not confuse them with assigning constraints in the scheduling tool.
- Modification: Discretionary dependencies can be modified or removed by the project team, unlike mandatory dependencies.
Typical PMI Exam Example
A project manager is developing the project schedule for a software development project. The team decides that testing should begin only after all coding is complete, based on industry best practices. However, the team recognizes that testing could theoretically begin on completed modules while coding continues on others. What type of dependency is this?
Answer: Discretionary dependency (preferred logic/soft logic)
PMI Exam Traps
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Trap: Confusing discretionary dependencies with mandatory dependencies when both could work Reality: Mandatory dependencies are required by law, contract, or physical nature. Discretionary dependencies are preferences based on good practices.
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Trap: Believing discretionary dependencies cannot be changed Reality: The project team determines which dependencies are discretionary and can modify or remove them during sequencing.
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Trap: Treating "good practice" sequences as mandatory Reality: Just because a sequence is recommended does not make it mandatory. The team can choose other sequences if beneficial.
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Trap: Confusing discretionary dependencies with external dependencies Reality: External dependencies involve relationships with non-project activities outside the team's control. Discretionary dependencies are internal choices based on good practices.
Important PMI Connections
| Related Concept | Relationship Type | Exam Attention Point |
|---|---|---|
| Mandatory Dependencies | Opposing concept | Distinguish: mandatory = required by law/contract/nature; discretionary = preferred but not required |
| External Dependencies | Related dependency type | External = outside project team control; discretionary = team choice |
| Activity Sequencing Process | Where dependencies are determined | The project team determines which dependencies are discretionary during sequencing |
| Schedule Constraints | Common confusion | Discretionary dependencies are NOT schedule constraints; constraints are assigned in scheduling tools |
| Project Risk Management | Impact relationship | Using sequential order for discretionary dependencies reduces overall project risk |
Quick Review Questions
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A construction project manager decides that electrical work should follow plumbing work based on industry good practices. Both activities could theoretically be done simultaneously. What type of dependency is this?
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During activity sequencing, the project team identifies that a prototype must be built before it can be tested due to physical limitations. What type of dependency is this?
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Can the project team modify or remove a discretionary dependency during the sequencing process?
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What is the primary difference between a discretionary dependency and a mandatory dependency?
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A software project has a dependency on hardware delivery from an external supplier. What type of dependency is this?
PMBOK v8 Reference
Section 6.3.2.2 - Sequencing Activities: Tools and Techniques (Dependency Types)
Note: This content is derived from the PMBOK v8 context provided and focuses exclusively on what is tested on the PMI exam regarding logic good practices in dependency determination.