When estimation in agile teams, the effort is often estimated in Story points. To make this work, the members in estimation team need to agree on the size of a Story point. This can be achieved by identifying a Reference story, a user story that all team members have common understanding its effort, risks, and complexity.
I have given a few estimation seminars internally within Tieto the last months. During the development of the training modules we used expertise from Magne Jorgensen, Simula Lab in Oslo. Magne’s research area is cost and effort estimation for software development. I learned from Magne that people will produce better estimates if they relate to some other known reference task that is medium/large than if the relate to a small reference task.
There is research by Magne with empirical studies that shows this. See below.
In the agile community, it is often recommended to identify a reference story to be 2 story points. All other user stories are then estimated relative to that reference story. E.g. a user story seems to be about 4 times as much work as the reference story. This would give 8 story points estimate.
I instead recommend teams to identify a medium/large reference story and give that 8 story points. The subsequent user stories will then be weighted against something a bit larger. There is a big advantage of this: Most estimation items will be similar or smaller in size as the reference story. The result is better (more accurate) estimates.
See the research paper: Relative Estimation of Software Development Effort: It Matters With What and How You Compare. There are some other interesting results and suggestions in the paper. Definitely recommended reading if you are interested in estimations.