Here is a summary of an article on how to use SPC to reduce varition of your team velocity. SPC charts are widely used in Six sigma projects in the analysis and control phase to reduce process variation.

How to Calculate and Use Velocity?

In scrum methodology velocity is the amount of work done during a sprint. Scrum teams use velocity to predict and plan how much scope can be delivered over the upcoming sprints.

For example if a team delivers within a specific sprint a four  user stories with a sum of story points of 23, the velocity is 23. let say in the next sprint print the same team delivers 27 story points, then the average velocity is 25, or (23 story points + 27 story point) divided by 2 sprints = 25.  So how do you control varition in the same team velocity. The missing part on the velocity chart is having control over the velocity variations! Velocity variations are those factors that may influence team velocity. So if you what to control the velocity variations over the velocity average, to accomplish that you need to use technique called Statistical Process Control (SPC) Chart.

Statistical Process Control (SPC) chart is a technique used to determine the stability and predictability of a process, in our case team velocity. SPC charts represent both the average and the process deviation to specify the upper and lower control limits.