Learn in the textbookEstimating Development Effort
DefinitionRelative estimation (相対見積) does not estimate work directly in time; instead it sizes a task by how many times as large it is as a chosen reference task.
ExplanationYou pick one reference task, then compare each other task with it, as a multiple or a fraction, and give it a value. In agile development this ratio is usually expressed in story points, a unit kept separate from time. It rests on the idea that people judge more easily by comparing two things than by naming an amount outright.
AnalogyIt is like skipping the scale and holding one bag in each hand to decide that this one is about twice as heavy.
Memory hookThe opposite of relative is absolute, the same pair as grading on a curve versus grading against a fixed standard.