Activity Duration Estimation: Work Effort and Resource Calendars

PMBOK v8 Definition

Estimating effort and duration requires an estimation of the amount of work effort required to complete the activity as well as the estimated number of available resources. These estimates are used to approximate the number of work periods (activity duration) needed to complete the activity using the appropriate project and resource calendars. The duration estimate is progressively elaborated, and the process considers the quality and availability of the input data.

Why It Matters for the Exam

This concept appears frequently in PMI exam questions testing your ability to distinguish between effort (person-hours) and duration (calendar time). Expect situational questions where you must calculate duration given effort and resources, or identify which estimation technique applies. Questions often appear in the Schedule Management Knowledge Area.

Key Points to Remember (for the exam)

Definition: Duration = number of work periods needed to complete an activity with estimated resources
Core Inputs: Scope of work, required resource types/skill levels, estimated resource quantities, resource calendars
Key Distinction: Effort ≠ Duration. Effort is person-hours; duration is calendar time
Progressive Elaboration: Duration estimates improve as more detailed and precise data becomes available
Influencing Factors: Constraints on duration, effort involved, type of resources (fixed duration, fixed effort, fixed number of resources), team experience, and risk involved
Source of Estimates: Person or group on the project team most familiar with the nature of the work for the specific activity
Key Techniques: Analogous estimating (gross value, less costly, less accurate) and Parametric estimating (statistical relationship, e.g., quantity × labor hours per unit)

Typical PMI Exam Example

Your project requires 400 hours of effort to complete a design activity. You have 2 designers available full-time (8 hours/day) on the project calendar. What is the estimated duration? Answer: 400 hours ÷ (2 resources × 8 hours/day) = 25 work days. The PMI exam tests this calculation with resource calendars and work periods.

PMI Exam Traps

Trap: Confusing effort with duration Reality: Effort is total person-hours; duration is calendar time including resource availability and calendars
Trap: Assuming analogous estimating is always accurate Reality: Analogous duration estimating is less costly and less time-consuming but also less accurate than other techniques
Trap: Ignoring resource calendars when calculating duration Reality: Duration uses both the estimated work effort AND the appropriate project and resource calendars
Trap: Thinking duration estimates are fixed at project initiation Reality: Duration estimates are progressively elaborated as input data quality and availability improve

Important PMI Connections

Related Concept	Relationship Type	Exam Attention Point
Resource Calendars	Input to duration estimation	Determines availability of resources for work periods
Scope Performance Domain	Source of scope of work	Provides the work content requiring effort estimation
Resources Performance Domain	Source of resource types/skill levels	Determines which resources are available and their quantities
Schedule Network Analysis	Technique using duration estimates	Duration estimates feed into critical path and schedule calculations

Quick Review Questions

What is the difference between effort and duration in project management?
A project has 600 hours of effort with 3 resources available 6 hours/day each. What is the estimated duration?
Which estimation technique uses a statistical relationship between historical data and other variables (e.g., square footage in construction)?
Why are duration estimates progressively elaborated throughout a project?
Who should provide the inputs for estimating duration of a specific activity?

PMBOK v8 Reference

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