Carl Bruiners Agile / IT Development Consultant

Improving Sprint capacity planning – The safety buffer / net

If your team consistently struggles to meet their sprint commitments because they are over optimistic with their story estimations, try to create a safety buffer or safety net.

Often teams try to create a safety buffer by only filling their sprint backlog to 60-80% of their capacity, though this approach is reasonable it is not very flexible or accurate.

Instead if we measure the uncertainty around estimated story points and use this metric to determine how much of a safety buffer we need to put into place, this gives us tighter accuracy and improved burn rates.

To demonstrate this (please note this excludes any JDI, BUA, etc..);

Team A has a velocity of 20 pts per sprint, they are planning to include stories made up of ten 1pt stories and five 2 pt stories. In this case we know that for the team to know an item is a 1pt or 2pt story they must have a clear idea of what is required to get the job done and the cone of uncertainty is virtually nothing. Based on the size of the stories I would recommend either no buffer or a very, very small one.

Team B also has a velocity of 20 pts per sprint, but their stories are individually larger in size, they have a 10pt, 5pt, 3pt and two 1pt stories. As there is a element of uncertainty surrounding the 10pt, 8pt and 5pt stories, I would recommend that the team builds in a greater safety buffer than Team A as there is an increased chance of there being more unknowns that will occur whilst the team is working on these larger stories.

How do we determine what the size of a safety buffer should be based on the stories being considered for a Sprint? This would require the Scrum Master to keep track of the teams points estimations and then at the end of a sprint note how big an item of work actually was, you then can use this figure as a multiplier for future stories of the same size.

For example if when a team plans a 10pt story, they actually take 12 points worth of effort in Sprint 1, then in Sprint 2 they take 14pt’s to deliver a 10pt story you have a multiplier of 1.3 for a 10pt story. If the team selects a 10pt story in a future Sprint your safety buffer would be 3pts.