One of key metrics of a Scrum team is Velocity. Even though it’s not mentioned in the Scrum guide, it is still very useful for the Product Owner while planning the Scope of the next Sprint and defining a concrete Sprint Goal.
As the name implies Velocity (consider the team as a vehicle) is a metric calculated by averaging the total amount of Story Points completed over the number of Sprints within the same period. In practice, the velocity of the teams gets stabilized after a certain number Sprints are left behind. As the number of Sprints increases, the velocity starts oscillating in a regular rhythm.
For instance, the velocity of our team is around 30 Story Points per Sprint. I’m confident that in normal circumstances our team is capable of bringing PBIs estimated as a total of 30 SPs to the DONE state within a Sprint. This is really an important asset I have in my pocket at the Sprint Planning session. This metric also makes Roadmap planning that encapsulates the following Sprints more reliable.
There is also another important term which is the Capacity. Capacity is how much availability the team has for the Sprint. This value is likely to vary from one Sprint to other. As an example from our team, the capacity of the team has reduced lately as one of the team members had to work part-time on another project, another one is about to leave the company.
Generally speaking, Velocity provides a guidance to the PO for the Sprint Planning about the total number of SPs the team will likely to complete. This point of view assumes that the capacity is not much different than the previous ones. Nevertheless, if the capacity is expected to be less, then the team could prefer to take less Product Backlog Items to the Sprint scope. At the end, it’s a kind of negotiation between the PO and development team. If there are strict deadlines, PO may need to push the team to perform overcapacity.
In a nutshell, velocity is a key metric that makes it easier for the PO to perform effective Sprint Planning. Besides, one should also take into account the capacity of the team as part of the team may not be 100% available during the Sprint.