3 Point Estimate
No Change
assess
3 Point Estimate is a technique we assess in the garden.
Summary
When to use:
- Teams stuck in Scrum story-point debates or long Planning Poker sessions without better forecasts.
- Stakeholders who need a range (68%, 90%, 95%) rather than one total.
- Engineering projects where limited information still requires a defensible rollup.
When to skip:
- The team will not maintain three numbers per task.
- Precision would be theater without tracking actuals against forecast.
| Approach | Tradeoff |
|---|---|
| Planning Poker | Fast consensus on one m value; weak on uncertainty |
| Story points | Common in Scrum; often debated without probabilistic rollup |
| Three-point | More fields per task; project-level confidence from task SD rollup |
If your team struggles with estimating, three-point ranges can reduce over-analysis compared to arguing a single effort number. Many teams can estimate dozens of tasks in the time it took to estimate a few with Planning Poker.
Details
Three Points Per Task
For each task, track three values:
a- The best case. This is probably what an expert would say. Or someone outside of engineering would think it should take.m- The likely case. This is what you would arrive at with Planning Poker or another consensus technique.b- The worst case. This is the “it could possibly take more than…” number.
The Math
For each estimated task, calculate the weighted average and standard deviation:
- $E = \frac{a + 4m + b}6$
- $SD = \frac{b-a}6$
Roll up to the project:
- $E(project) = \sum E(task)$
- $SD(project) = \sqrt{\sum SD(task)^2}$
Confidence intervals:
- 68% = $E(project) \pm SD(project)$
- 90% = $E(project) \pm 1.645 \times SD(project)$
- 95% = $E(project) \pm 2 \times SD(project)$
- 99.7% = $E(project) \pm 3 \times SD(project)$
95% is usually the target.
With a spreadsheet, rollup and intervals are straightforward.