For big tasks I use trivariate estimates - three numbers: best‑case, nominal‑case, and worst‑case. Each number is a confidence estimate:
- Best‑case (5%): only 5% of similar tasks finish this fast.
- Nominal‑case (50%): about half the tasks finish by this time.
- Worst‑case (95%): 95% confidence the task finishes by this time.
For example: “I’m 5% sure we’ll finish in 1 week, 50% sure in 2 weeks, and 95% sure in 3 weeks.” That means out of 100 similar tasks, ~5 finish in 1 week, ~50 in 2 weeks, and ~95 in 3 weeks.
Trivariate estimates are great for long‑term planning and showing uncertainty clearly. They let stakeholders make risk‑aware decisions. But they are usually too broad for day‑to‑day work inside a project. For short‑term planning and team flow we can use another approach, story points.
Discussions for your team
- Which projects need trivariate forecasts vs. simpler ranges?
- How often should we update the three numbers as we learn more?
- How do we present these to stakeholders so they understand risk?