Architecture & practice·Estimation and planning
instead of estimating the migration, you looked at how long the last three migrations actually took.
Reference-class forecasting
Also calledoutside view, historical forecasting
Estimating by comparison to how long similar past work really took, rather than by decomposing the work in front of you. It is markedly more accurate than bottom-up estimation because it silently includes all the interruptions, rework and forgotten steps that never appear in a breakdown. Its requirement is data: a team that never records how long anything took cannot use the outside view, which is a cheap thing to start collecting.