Estimation: An Approximation That Narrows with Information, Not Certainty
An estimate is an approximation carrying uncertainty, not a certain number.
- Wide early estimate — Little information and low accuracy
- Narrowing with information — Accuracy improves gradually
- More accurate estimate — Fewer unknowns closer to execution
Estimation estimates the time and cost the work needs. Techniques include: analogous, relying on similar projects; parametric, using a formula linking quantity to a rate; bottom-up, aggregating component estimates; and three-value using optimistic, pessimistic, and most likely values to account for uncertainty. Estimate accuracy narrows as the project progresses and information becomes available. An estimate is an approximation, not certainty, so it is reviewed and updated.
Treating an estimate as a certain number. An estimate is an approximation carrying uncertainty and is updated as information becomes available.
Estimate as a range, not a single number, and update as you know more.
Ready to start serious PMP prep?
Subscribe now →Read the details
Estimation is a core planning skill, but it is an approximation, not a definitive truth. Its techniques vary by available information and required accuracy: analogous is fast and relies on experience from similar projects but is less accurate; parametric uses a mathematical relationship between work quantity and a known rate, giving higher accuracy when reliable data exists; bottom-up aggregates estimates of the smallest components, being the most accurate and most effortful. To account for uncertainty, a three-value estimate uses optimistic, pessimistic, and most likely values, producing a range instead of a single number. Accuracy improves gradually as the project progresses and unknowns shrink, so the estimate is reviewed and updated periodically, not once.
Frequently asked questions
What is analogous estimation?
Estimation based on data from similar past projects; fast and less accurate.
What is parametric estimation?
Estimation using a formula linking work quantity to a known rate; more accurate with reliable data.
What is bottom-up estimation?
Aggregating estimates of the smallest components; the most accurate and most effortful.
What is a three-value estimate?
Using optimistic, pessimistic, and most likely values to account for uncertainty and produce a range.
Why does an estimate improve over time?
Because information increases and unknowns shrink as the project progresses.