Three-Point Estimating (PERT)
Three-point estimating uses three estimates (optimistic, most likely, and pessimistic) to define an approximate range for an activity's duration or cost, improving accuracy by considering estimation uncertainty.
Explanation
Three-point estimating, often associated with the Program Evaluation and Review Technique (PERT), addresses the inherent uncertainty in single-point estimates by considering three scenarios. The optimistic estimate (O) represents the best-case duration assuming everything goes well. The most likely estimate (M) represents the duration given normal conditions and expected resources. The pessimistic estimate (P) represents the worst-case duration if significant problems occur.
Two common formulas are used to calculate the expected duration. The triangular distribution uses a simple average: (O + M + P) / 3. The beta distribution (PERT) weights the most likely estimate more heavily: (O + 4M + P) / 6. The beta distribution is more commonly used on the PMP exam and produces an estimate that is more weighted toward the most likely outcome.
The standard deviation of a PERT estimate is calculated as (P - O) / 6, and the variance is the square of the standard deviation: [(P - O) / 6]^2. These values are used in quantitative risk analysis and to determine confidence intervals for the project schedule. A wider spread between optimistic and pessimistic estimates indicates greater uncertainty in the activity duration.
Key Points
- •Uses three estimates: Optimistic (O), Most Likely (M), Pessimistic (P)
- •Beta/PERT formula: (O + 4M + P) / 6
- •Triangular formula: (O + M + P) / 3
- •Standard deviation: (P - O) / 6; Variance: [(P - O) / 6]^2
Exam Tip
Memorize both formulas. The beta (PERT) formula (O + 4M + P) / 6 is more commonly tested. Know how to calculate standard deviation and variance for schedule risk questions.
Frequently Asked Questions
Related Topics
Estimate Activity Durations
Estimate Activity Durations is the process of estimating the number of work periods needed to complete individual activities with the estimated resources.
Analogous Estimating
Analogous estimating uses historical data from similar past activities or projects as the basis for estimating the duration or cost of a current activity or project.
Parametric Estimating
Parametric estimating uses a statistical relationship between historical data and other variables to calculate an estimate for activity parameters such as cost, budget, and duration.
Bottom-Up Estimating
Bottom-up estimating is a method of estimating project duration or cost by aggregating the estimates of the lower-level components of the work breakdown structure.
Most-studied PMP concepts
High-yield topics our learners drill most before exam day.
Resource Leveling
Resource leveling is a resource optimization technique in which adjustments are made to the project schedule to keep resource usage at or below a defined limit, often resulting in a longer project duration.
Resource Smoothing
Resource smoothing is a resource optimization technique that adjusts activities within their available float so that resource requirements do not exceed predefined limits, without changing the project end date.
Burndown Chart
A Burndown Chart is a graphical representation of work remaining versus time in a Sprint or release, showing whether the team is on track to complete the planned work.
Risk Register
The risk register is a project document that records the details of individual project risks, including their identification, analysis results, response plans, and current status.
Stakeholder Mapping
Stakeholder mapping is the visual representation of stakeholder relationships, influence, interest, or other attributes using grids, matrices, or diagrams to support analysis and engagement planning.
Relative Estimation
Relative Estimation is an agile technique where work items are sized in comparison to each other rather than in absolute units like hours or days, providing faster and more accurate estimates.
Cost Performance Index (CPI)
Cost Performance Index (CPI) is an EVM efficiency metric that measures cost performance as the ratio of earned value to actual cost: CPI = EV / AC.
Schedule Performance Index (SPI)
Schedule Performance Index (SPI) is an EVM efficiency metric that measures schedule performance as the ratio of earned value to planned value: SPI = EV / PV.
Earned Value Management (EVM)
Earned Value Management (EVM) is a methodology that integrates scope, schedule, and cost data to assess project performance and progress objectively.
Part of
Schedule Management
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