Critical Path Method: Drag, Drag Cost, and Buffer Protection Index

PMBOK v8 Definition

The Critical Path Method (CPM) is a method used to estimate the minimum project duration and determine the amount of scheduling flexibility (float) on the logical network paths within the schedule model. Critical path drag is the amount of time that an activity on the critical path is adding to the project duration, or alternatively, the maximum amount of time one can shorten the activity before it is no longer on the critical path or before its duration becomes zero. Critical path drag cost is the amount by which a project’s expected return on investment (ROI) is reduced due to the critical path drag of a specific critical path activity—the true economic cost comprised of its resource cost plus the cost of delay.

Why It Matters for the Exam

Critical path concepts appear frequently in PMI exam questions on Schedule Management, particularly in questions testing your ability to identify which activities directly impact project duration and prioritize tasks to protect the due date. Expect scenario-based questions where you must calculate drag, identify the activity with the highest drag cost, or distinguish between CPM and Critical Chain Project Management (CCPM) metrics.

Key Points to Remember (for the exam)

• Critical Path Drag Definition: The amount of time a critical path activity adds to project duration; also the maximum time you can shorten it before it leaves the critical path or reaches zero duration.
• Critical Path Drag Cost: The reduction in project ROI caused by a specific critical path activity; equals resource cost plus cost of delay (foregone acceleration premium).
• Critical Path Method (CPM): Estimates minimum project duration and determines float on logical network paths.
• Critical Chain vs. Critical Path: CCPM uses resource-loaded and resource-leveled critical path (RLCP), which is synonymous with "the critical chain."
• Buffer Protection Index (BPI): CCPM metric that compares percentage of project buffer remaining with percentage of critical chain remaining; designed to protect the project due date.
• BPI vs. SPI: BPI protects the due date; SPI (from EVM) gauges how far from the schedule baseline the project may be.
• CCPM Buffer Rule: A critical chain buffer is in lieu of task-level or sprint-level buffers, not in addition to them.

Typical PMI Exam Example

Your project has three critical path activities: Activity A (10 days, drag cost $5,000/day), Activity B (8 days, drag cost $2,000/day), and Activity C (6 days, drag cost $1,000/day). The project is at risk of missing its due date. Which activity should you prioritize for compression? Answer: Activity A, because it has the highest critical path drag cost ($5,000/day), representing the greatest reduction in project ROI.

PMI Exam Traps

• Trap: Confusing critical path drag with total float.

  • Reality: Drag applies only to critical path activities (zero float); total float is for non-critical paths.

• Trap: Thinking BPI and SPI measure the same thing.

  • Reality: BPI protects the due date; SPI measures deviation from the schedule baseline.

• Trap: Assuming CCPM buffers are added on top of task-level buffers.

  • Reality: CCPM buffers are in lieu of task-level or sprint-level buffers, not in addition.

• Trap: Confusing critical path drag cost with activity resource cost alone.

  • Reality: Drag cost = resource cost plus cost of delay (foregone acceleration premium).

Important PMI Connections

Quick Review Questions

1. What is the difference between critical path drag and critical path drag cost?
2. If a critical path activity can be shortened by 5 days before it leaves the critical path, what is its drag?
3. How does the Buffer Protection Index (BPI) differ from the Schedule Performance Index (SPI)?
4. In CCPM, is the project buffer added on top of task-level buffers or in lieu of them?
5. Which metric should you use to prioritize day-to-day tasks to protect the due dates of projects most at risk?

PMBOK v8 Reference

Section 5 – Tools and Techniques (pages 159-160)