Story Points are an abstract, arbitrary number associated with User Stories that indicates the relative effort (not complexity) required to complete the story development.
Often used in the form of a Fibonnacci series (1, 2, 3, 5, 8, 13, 21, 34, ...) or a simple integer value Story Points are intended to indicate relative effort, not complexity, of low level requirements. Story Points are an estimation method based on picking a well-understood "normal" story, setting its value and then estimating other stories relative to the "normal"story.
Points are an abstract team based indicator and are not comparable across teams. They do not equate to "person-days" or complexity and so are fundamentally unsuitable for use in contractual arrangements.
Points cannot be aggregated meaningfully across teams or up to programme level due to their arbitrary team based value. For this reason,we strongly recommend against, using points at both a Programme Backlog and Product Backlog level.
Story Point estimating may be useful within a team to help size Stories for inclusion or not in sprints/iterations. This is an in-team private metric that does not make sense outside of the team. Over time story point sizes tend to lower values as large stories are broken up more is understood about the work (risks reduce in line with the Cone of Uncertainty). As a result, points are not numerically consistent even within the context of a single team over time. We’ve seen many planning and reporting dysfunctions based on poor understanding, and the implied false accuracy of Story Points such as Project Managers setting a target velocity!
How quickly are we working? When will we be done?
Velocity is how many points are completed over time and is often used, especially in Scrum teams, as a measure of progress towards the total number of points.
Over time teams will gain an understanding of roughly how many points they can deliver in a period of time (or an iteration/sprint). This is called their "Velocity" and can be used to extrapolate remaining effort to completion (ETC). This is the original intention behind story points and is why they do not equate to complexity since some very complex things don't take long to implement but some very simple (but large in volume) requirements can take a long time to implement. When using velocity it will normally vary a little over the lifecycle and will typically be a little unstable during the first couple and last few time periods/iterations/sprints.
As with any estimate, we recommend presenting it with an Uncertainty Indicator. Extrapolation of progress as effort indicators based on actual activity so far helps teams to answer "how long until it's done?". For more on tracking Story Points over time see Workflow Metrics.
- Abstract numbers are difficult for people and leaders to understand
- Their meaning changes over time as the Cone of Uncertainty is reduced in teams undermining extrapolation
- They imply false accuracy by being, often small discrete numbers
- They offer no additional benefit over simply counting items complete
- They cannot be aggregated because they are team-defined and not normalized against any standard
We recommend strongly resisting schemes to normalize story points in organizations, or make the same mistakes with other purely abstract measures (such as Business Value Points!)