Activity Duration Number: Estimating Work Periods for Completion

PMBOK v8 Definition

The activity duration number is the estimated number of work periods (hours, days, weeks, or other) needed to complete individual activities with estimated resources. Estimating effort and duration uses information from the scope of work, required resource types or skill levels, estimated resource quantities, and resource calendars to approximate the number of work periods (activity duration) needed to complete the activity using the appropriate project and resource calendars.

Why It Matters for the Exam

This concept appears frequently in PMI exam questions related to Schedule Management and Resource Management. Expect scenario-based questions where you must determine how resource availability, skill proficiency, and work constraints affect duration estimates, as well as questions testing the relationship between effort, duration, and resources.

Key Points to Remember (for the exam)

• Core Formula: Duration = Effort ÷ Number of Resources (adjusted for productivity factors)
• Key Inputs: Scope of work, resource types/skill levels, estimated resource quantities, resource calendars
• Critical Factor: Number of resources expected to be available AND their skill proficiency levels determine activity duration
• Progressive Elaboration: Duration estimates improve as more detailed and precise data become available
• Fixed Constraints: Some activities have intrinsic duration constraints (e.g., 24-hour stress test) regardless of resource allocation
• Diminishing Returns: Doubling resources does NOT halve duration; adding too many resources may increase duration due to knowledge transfer, learning curves, and coordination overhead
• Technology Impact: Advances in technology can significantly impact duration by changing output capacity

Typical PMI Exam Example

A project team estimates a software testing activity requires 120 hours of effort. Two testers are available, each working 8 hours per day. What is the estimated activity duration in work days? (Answer: 120 ÷ (2 × 8) = 7.5 days). However, if testers have different skill levels, the duration may increase due to training needs.

PMI Exam Traps

• Trap: Confusing effort with duration

  • Reality: Effort is person-hours of work; duration is calendar time. They are NOT interchangeable.

• Trap: Assuming doubling resources halves duration

  • Reality: Adding resources creates coordination overhead, knowledge transfer needs, and learning curves that reduce efficiency gains.

• Trap: Forgetting skill proficiency impacts duration

  • Reality: The number of resources AND their skill levels both determine activity duration; a change to a driving resource influences duration.

• Trap: Applying resource estimation rules to fixed-duration activities

  • Reality: Some activities (e.g., stress tests, curing time) have predetermined durations regardless of resource allocation.

Important PMI Connections

Quick Review Questions

1. A project requires 200 hours of effort for a task. If 4 resources are available but only 2 have the required skill proficiency, what is the estimated duration in days (assuming 8-hour workdays)?

2. Why does doubling the number of resources NOT always reduce activity duration by half?

3. What type of activity has a predetermined duration that cannot be shortened by adding more resources?

4. Which inputs are used to estimate activity duration according to PMBOK v8?

5. How does progressive elaboration affect the accuracy of duration estimates?

PMBOK v8 Reference

Section 2.6 - Resources Performance Domain (Step 3: Estimate effort and duration)