Duration Estimating: Law of Diminishing Returns and Resource Factors

PMBOK v8 Definition

Duration estimating involves approximating the number of work periods needed to complete an activity using appropriate project and resource calendars. Key factors include the law of diminishing returns, where increasing one factor (e.g., a resource) while keeping others fixed eventually yields progressively smaller increases in output, and the number of resources available with their skill proficiency levels, which may determine the activity's duration. Some work has an intrinsic nature that takes a predetermined amount of time regardless of resource allocation (e.g., a 24-hour stress test).

This concept is part of the Schedule Management Knowledge Area, specifically within the Estimate Activity Durations process.

Why It Matters for the Exam

The PMI exam frequently tests your understanding of what truly influences activity duration versus common misconceptions. Questions often present scenarios where adding more resources seems like the obvious solution, but the correct answer involves recognizing constraints like the law of diminishing returns or fixed duration work. This appears in situational questions about schedule compression, resource optimization, and realistic duration estimation.

Key Points to Remember (for the exam)

• Main Input: Project management plan (any component), project documents (assumption log, basis of estimates, milestone list)
• Main Output: Duration estimates, basis of estimates
• Key Tool: Analogous estimating, parametric estimating, three-point estimating, bottom-up estimating
• Law of Diminishing Returns: Adding more resources beyond an optimal point reduces productivity per additional resource
• Fixed Duration Work: Some activities have predetermined durations that cannot be shortened by adding resources (e.g., curing concrete, stress tests)
• Resource Skill Levels: The proficiency of available resources directly impacts activity duration, not just the count of resources
• Common Confusion: Confusing "effort" (person-hours) with "duration" (calendar time) – they are not interchangeable

Typical PMI Exam Example

A project manager is estimating the duration for a concrete curing activity. The team suggests adding more workers to complete it faster. What should the project manager consider?

Correct analysis: Concrete curing is an intrinsic constraint that takes a predetermined amount of time regardless of resource allocation. Adding workers will not reduce the duration; it only increases cost without schedule benefit.

PMI Exam Traps

• Trap: Believing that adding more resources always reduces duration

• Reality: The law of diminishing returns means after an optimal point, additional resources yield smaller increases in output

• Trap: Assuming all activities can be compressed with more resources

• Reality: Some activities have fixed durations (e.g., curing, testing, approvals) that are independent of resource count

• Trap: Confusing "effort" with "duration" when estimating

• Reality: Effort is person-hours worked; duration is calendar time. They are related but not equivalent

• Trap: Ignoring resource skill proficiency when estimating

• Reality: The number of resources AND their skill levels both determine duration, not just headcount

Important PMI Connections

Quick Review Questions

1. A project has a 48-hour stress test for a critical system. The sponsor demands completion in 24 hours by adding more testers. What should the project manager explain?

2. During duration estimation, a team member proposes adding 10 more developers to a software coding task that already has 5 developers. Which concept should the project manager consider?

3. Two activities require the same effort (100 person-hours). Activity A has 2 senior developers, Activity B has 4 junior developers. Which activity likely has a shorter duration and why?

4. A project manager estimates an activity will take 10 days with 2 resources. The sponsor asks for a 5-day estimate with 4 resources. What assumption is the sponsor making that may be incorrect?

5. During a retrospective, the team discovers that adding more testers to a quality inspection actually increased the time to complete. Which PMBOK concept explains this phenomenon?

PMBOK v8 Reference

Section 6.4 - Estimate Activity Durations (Schedule Management Knowledge Area)

The law of diminishing returns and number of resources are explicitly listed as factors to consider when estimating duration. The concept of intrinsic work constraints (e.g., 24-hour stress test) is also documented as a factor that takes predetermined time regardless of resource allocation.