Velocity, Work, and Time
Velocity = Work / Time
For example, a four-member team might have a velocity of 20 points per week or 80 points per sprint.
Time
To maintain a continuous pace of progress, agile development uses one of two tactics:
- Iterative increment: maintains the pace through sprint pulses. It uses the sprint as the unit of time and expresses velocity as work done per sprint. Technical Scrum uses iterative increment.
- Continuous increment: maintains a steady flow without idle time or bottlenecks. There are no sprints, and velocity is expressed in points per week, day, or month.
Elapsed time and ideal time
It is important to know the difference between the two:
- Elapsed time: working time equivalent to the working day. For a team of four people working eight-hour days, the elapsed time in one week is 4 × 8 × 5 = 160 hours.
- Ideal time: working time under conditions of full concentration, with no breaks for interruptions, unrelated matters, or context switching. It is used in estimates as the unit of effort required. Example: "that task has a size of 3 ideal hours."
Ideal time is always less than elapsed time. A basketball game lasts 40 minutes of exact playing time, but the actual game usually runs over an hour. Similarly, a task that takes "one ideal hour" may take up half a working day in elapsed time. Managing this difference helps organize work more objectively and avoid the stress of unattainable goals.
Work
Measuring work is necessary for two reasons: to record the work already done and to estimate the work still to be done.
Work already done
Agile management does not determine the degree of project progress by the effort expended, but by the work still to be done. Other processes in the organization may need to record the effort invested, but it should not be used to calculate project progress: a team may have worked hard and made little progress if the work does not translate into a done increment.
Work still to be done
Scrum measures the remaining work to:
- Estimate the expected effort and time to complete user stories and epics.
- Determine the progress of the project and of each sprint.
Units of work
A piece of work can be sized by measuring the product being built or the time it takes to do it. In agile management, "points" or "story points" are commonly used as a relative unit of work. Each organization institutionalizes its own "point." It can be defined as:
- The relative size of a known task. Example: in an online store team, "one point" equals the size of a user invoice listing.
- Based on the ideal time required. Example: "one point equals four ideal hours of work."
What matters is that the definition of the "point" is consistent across all of the organization's measurements and known to everyone.
Velocity
Velocity in technical Scrum is the amount of work done by the team in a sprint. A velocity of 150 points indicates that the team completes 150 points of work per sprint. In pragmatic Scrum, where sprints may have different lengths or the team size varies, velocity is expressed by stating the unit of time and whether it refers to the whole team or the average per person:
- "The team's average velocity is 100 points per week."
- "The average velocity per person is 5 points per day."
Common mistake
Using elapsed time as the estimation unit instead of ideal time. Estimating a story at "eight hours" on the assumption that they are eight hours of the working day usually leads to unrealistic commitments: meetings, interruptions, context switching, and other tasks consume a significant part of those eight hours. Estimating in ideal time and applying a realistic conversion factor when planning the sprint is more accurate and causes the team less frustration.
See also
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