Three-Point Estimation
An estimation technique that uses optimistic, most likely, and pessimistic values to calculate a weighted expected effort.
Also known as: PERT Estimation, Three-Point Estimate, Program Evaluation and Review Technique
Category: Techniques
Tags: estimation, planning, project-management, statistics, risk-management
Explanation
Three-Point Estimation is a technique that produces more realistic estimates by explicitly accounting for uncertainty. Instead of a single number, the estimator provides three values and uses them to calculate a weighted average.
## The three points
- **Optimistic (O)** - The best-case scenario if everything goes right
- **Most Likely (M)** - The expected effort under normal conditions
- **Pessimistic (P)** - The worst-case scenario if things go wrong (but the project is still completed)
## Formulas
**Triangular distribution (simple average):**
E = (O + M + P) / 3
**PERT-weighted (Beta distribution):**
E = (O + 4M + P) / 6
The PERT formula gives four times the weight to the most likely estimate, producing results closer to the expected outcome under normal conditions.
**Standard deviation:**
SD = (P - O) / 6
The standard deviation quantifies the uncertainty in the estimate and can be used to calculate confidence intervals.
## Example
Estimating a feature:
- Optimistic: 3 days
- Most Likely: 5 days
- Pessimistic: 15 days
PERT estimate: (3 + 4×5 + 15) / 6 = 38/6 = **6.3 days**
SD: (15 - 3) / 6 = **2 days**
This means there's roughly a 68% chance the task takes between 4.3 and 8.3 days, and a 95% chance it takes between 2.3 and 10.3 days.
## When to use it
- **High-uncertainty tasks** - When outcomes could vary significantly
- **Stakeholder communication** - Presenting ranges rather than false precision
- **Risk analysis** - Identifying tasks with the widest pessimistic-optimistic gaps
- **Project-level aggregation** - Summing estimates and standard deviations across tasks
- **Contract negotiation** - Justifying timelines with statistical backing
## Advantages
- Forces consideration of both best and worst cases
- Produces a range that communicates uncertainty
- Statistical foundation allows aggregation across tasks
- Reduces the planning fallacy by requiring pessimistic scenarios
## Limitations
- People tend to underestimate pessimistic values
- More time-consuming than relative estimation methods
- Assumes a specific probability distribution (which may not match reality)
- Individual estimates can still be subject to cognitive biases
Three-point estimation originated from the PERT (Program Evaluation and Review Technique) developed by the U.S. Navy in the 1950s for the Polaris missile program.
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