The core reasoning behind not allowing relationships between WBS (Work Breakdown Structure) elements in tools like Primavera P6 and Planera, as opposed to Microsoft Project, boils down to clarity, structural integrity, and separation of concerns between organizational hierarchy and logical scheduling. Here’s a deeper breakdown:
1. WBS Is an Organizational Structure, Not a Schedule Logic Tool
WBS defines what is being done — it’s a hierarchical representation of scope, grouping activities into manageable chunks.
Relationships (dependencies) define how and when things happen — the logical sequence of activities.
Mixing the two creates conceptual confusion. If relationships are allowed between WBS nodes, it blurs the line between scope hierarchy and schedule logic.
2. Encourages Activity-Level Precision
By restricting relationships to activities (not WBSs), tools like P6 enforce granular, precise dependency modeling.
It ensures that planners explicitly define which tasks are dependent, rather than loosely connecting high-level scope areas.
This leads to more accurate critical path analysis and risk assessment.
3. Avoids Implicit Dependencies That May Be Misleading
A relationship between WBS elements might implicitly suggest that all child activities are dependent — but this may not be true.
Preventing WBS-level dependencies avoids accidental global constraints or false critical paths.
4. Prevents Circular Logic and Over-Constraining the Plan
Allowing WBS-to-WBS relationships can lead to complex, hard-to-debug logic loops.
Keeping relationships at the activity level ensures the plan remains deterministic and explainable.
5. Aligns with CPM Best Practices
Critical Path Method (CPM) planning best practices advocate for relationships to be established only between lowest-level tasks.
This avoids overgeneralized sequencing and ensures more reliable float calculations and scheduling outcomes.
Why MS Project Allows It, but it should not
Microsoft Project is too flexible, almost to a fault, often targeting less complex or smaller-scale projects.
Allowing WBS-like summary task dependencies gives planners a quick way to sequence phases — but this often results in overly simplistic or misleading logic and costly mistakes.
Advanced schedulers often recommend avoiding summary task links in MS Project for the same reasons mentioned above.
Conclusion
The decision to disallow WBS-to-WBS relationships in Planera and P6 is intentional. It enforces discipline in planning, keeps scope and logic clearly separated, and aligns with best practices for robust, scalable scheduling — especially in large, complex construction projects where such clarity is essential.