Why use Triangulation?

Mike Cohn, in his excellent book "Agile Estimating and Planning", teaches different mechanisms for estimating user stories. One such technique is to estimate by analogy (also referred to as Triangulation). When estimating by Triangulation, a team compares the user story they want to estimate with some previously estimated stories. They then decide if the user story is about the same size, smaller or bigger than the other estimated stories. As mentioned by Cohn in his book, there are strong evidence that humans are much better at estimating relative size than they are at estimating absolute size. So, it is much easier for a team to provide estimates by comparing user stories between them.

Additional benefits

Using Triangulation also has other benefits. Without a good reference system in place, teams sometimes start seeing almost everything as a one, two or three story points. Over time, the face value of a story point starts fluctuating and this causes the team's velocity to fluctuate too. Triangulation may also help the team during planning poker sessions. After each team member has revealed their personal estimate, the team is able to ground their collective estimate by comparing the story with similar work they executed in the past. If there are big discrepancies between the estimates, the team can review past user stories with the same number of story points and see which fits best. Triangulation is a very useful tool that should be in every team's utility belt!