Estimating Activity Durations

Estimating Activity Durations: A Comprehensive Guide for PMI-SP Exam

Why Estimating Activity Durations is Important

Estimating activity durations is a critical component of schedule management because:

• It forms the foundation for creating a realistic project schedule
• It determines when activities can start and finish
• It influences resource allocation and costs
• It helps identify the critical path
• It enables accurate timeline commitments to stakeholders
• It provides a baseline for monitoring progress and performance

What is Activity Duration Estimation?

Activity duration estimation is the process of approximating the number of work periods needed to complete individual activities with the estimated resources. It answers the question: "How long will each task take to complete?"
Duration estimates are:
• Expressed in work periods (hours, days, weeks, months)
• Based on the effort required and resources available
• Progressive elaborations that become more accurate as the project progresses
• Different from effort estimates (person-hours), which may not translate directly to duration

How Activity Duration Estimation Works

Inputs:
• Activity list from the activity definition process
• Resource requirements and availability
• Historical information and organizational process assets
• Resource calendars
• Project scope statement
• Risk register
• Resource breakdown structure
• Enterprise environmental factors

Techniques:

1. Expert Judgment: Using experience of subject matter experts

2. Analogous Estimating: Using durations from similar activities in previous projects
• Fast but less accurate
• Useful when limited information is available
• Top-down approach

3. Parametric Estimating: Using mathematical models based on historical data and parameters
• Example: 2 hours per module × 10 modules = 20 hours
• Can be very accurate if parameters are solid

4. Three-Point Estimates: Using weighted average of optimistic, most likely, and pessimistic estimates
• PERT formula: (O + 4M + P) ÷ 6
• Triangular Distribution: (O + M + P) ÷ 3
• Accounts for uncertainty and risk

5. Bottom-up Estimating: Breaking activities into smaller components, estimating each, then aggregating
• Most accurate but time-consuming
• Used for high-risk or complex activities

6. Reserve Analysis: Adding contingency reserves to account for schedule risk

7. Group Decision-Making Techniques: Using methods like Delphi, nominal group technique, etc.

Outputs:
• Activity duration estimates
• Basis of estimates (assumptions, constraints, ranges)
• Updates to project documents

Common Challenges in Duration Estimating

• Student Syndrome: Delaying work until the deadline approaches
• Parkinson's Law: Work expanding to fill the time available
• Padding: Adding extra time as hidden contingency
• Optimism bias: Underestimating complexity and risks
• Multitasking inefficiencies: Context switching costs
• Inadequate historical data: Making comparisons difficult

Best Practices for Accurate Duration Estimates

• Involve the people who will do the work
• Consider resource skill levels and experience
• Account for resource availability and productivity factors
• Use multiple estimation techniques for comparison
• Document assumptions and constraints
• Update estimates as more information becomes available
• Review and learn from past estimation accuracy
• Consider communication and coordination overhead
• Build in appropriate (but transparent) reserves

Exam Tips: Answering Questions on Estimating Activity Durations

1. Know the PMBOK terminology:
• Understand the difference between duration, effort, and elapsed time
• Distinguish between activity durations and milestone dates

2. Identify the appropriate estimation technique:
• Look for clues about available information (historical data, parameters, experts)
• Consider project phase and estimation purpose

3. Calculate using formulas correctly:
• Be prepared to apply PERT formulas: (O + 4M + P) ÷ 6
• Know how to convert effort hours to duration based on resources

4. Recognize estimation pitfalls:
• Be alert for questions about estimation bias and how to address it
• Understand when padding is inappropriate versus when contingency is appropriate

5. Remember the inputs and outputs:
• Know what information is needed for good estimates
• Understand how duration estimates feed into schedule development

6. Apply situational judgment:
• For scenario questions, consider the project context, constraints, and objectives
• Assess the impact of risks on duration estimates

7. Identify the role of historical information:
• Historical information influences all estimation techniques
• Lessons learned impact how you should approach estimation

Remember that for PMI-SP exam questions, you'll need to identify not just what you could do, but what you should do as a scheduling professional following PMI standards. Focus on selecting answers that demonstrate sound judgment and adherence to best practices in schedule management.