Velocity: What Does It Measure?

Driving Towards Agile Metrics

What can velocity tell us?

In my last two blog entries, I discussed scaling the Sprint to a roadmap and scaling the User Story to a full Requirements Breakdown Structure (RBS). I mentioned that I would be attaching “Metrics” to the RBS model as an entity hanging off Features.

Before moving on to a birds-eye view of Agile metric, I’d like to establish a shared understanding of a few other concepts.

In this entry, let’s discuss velocity.  In bringing up the topic of Agile metrics, I’ve often heard, “Metrics? Do you mean velocity?”  Yes, velocity is a numeric value which can be tracked, but what does it really measure?

Productivity or Predictability?

The Velocity Chart: Story Points per Sprint

Simply put, velocity doesn’t measure the productivity of a team.  It measures a team’s ability to accurately estimate its work.

What exactly is velocity?  It’s the number of Story Points of effort a team can accomplish in a Sprint.  As you probably know, Story Points are arbitrary units of measure used to “size” one Story relative to another.  Over time (usually 3 - 5 Sprints), a team comes to a shared understanding of what a Story Point is and how many it can consistently execute in one Sprint.

In the chart to the right, you’ll notice a wider fluctuation in the number of Story Points accomplished in Sprints 1 - 4. By Sprint 5, the team begins leveling out at about 16 Story Points per Sprint. What does this mean? It means that the team can estimate and deliver work more consistently.

Starting the Car

Coming up with an initial velocity estimate is similar to estimating how fast a race car can go.  You can invest in collecting and analyzing detailed data to develop your estimate from the bottom up, or you can take a leaner, heuristic approach.   What if you were to say, “Based on my 10 years of experience driving race cars, I think I can comfortably sustain a pace on this race track of 110 mph.”  To validate this estimate, you could then hop into the car, drive it around the track a few times, watch your actual velocity, and adjust your estimate based on real-time feedback.

Estimating Arrival Time

In this unpredictable world we are not all driving race cars on race tracks.  Is a car which can go 110 mph around a smooth track better than a pickup truck which can drive 80 mph over back roads?  It depends on business needs and market conditions.

So what’s the real value of knowing your velocity?  The value is in being able to consistently predict when you’ll arrive at your next destination.  And the next.  And the next.

There will always be anomalies: a key team member is sick with swine flu or a Microsoft critical update downs all database servers.  In such situations, the velocity for that Sprint can be footnoted.  It’s no more than a pot hole on a stretch of highway which blows a tire.  You fix it and keep moving.