This article aims to explain what are the main differences between Story Points and Function Points, and study their complementarity for agile projects.
Story Points are a measure unit resting on the perception of the work to be done by the project team. The determination of that size is based on the level of comprehension of the complexity, and thus, the required effort.
Story Points can be implemented more faster than Function Points. It is a collaborative approach involving the entire project team and that enables to share a unique comprehension of each user story, besides the respect of a schedule.
“Function Points measure software by quantifying the features compared to the final client needs, mainly based on its logical conception”.
Unlike Story Points, Function Points are a standard measure unit that is replicable. Features may be evenly measured in Function Points, regardless of who measures them. Function Points may be used for agile projects and non agile projects, but the project team will not be able to count them. Indeed, the counting must be executed by experts or an automated tool.
Story Points or Function Points?
The combined use of both methods can be useful. They both aim at the same goal: efficiently manage a software development project.
Function Points are especially accurate to get to global measure of a product, at the beginning and at the end of the lifecycle of a project. The measure in Function Points at the beginning of a project may be used to estimate the charge, the global costs and time. At the time of the launch of production, the Function Point Measure may be used to assess productivity and to refine the calibration of the effort models of estimation.
On the other hand, Story Points can efficiently manage le workflow of an agile project. They may be used to estimate the quantity of work that can done by a team in a delimited period of time.
Comparative table in the case of an agile project
|Goals||Function Points||Story Points|
|Initial budget estimate||Yes, but sometimes macro, since all the requirements are not defined yet||No, because the unit is specific for each project|
|Sprint content estimate||Yes||Yes|
|Productivity assessment||Yes||Yes, but relative, because the perception of Story Points changes form one team to another|
|Easy to measure||Difficult, because it requires the intervention of an expert or the use of an automated tool||Easy, can be performed by the team.|
|Comparison with other projects||Yes, since it is standard||No, since each measure is specific to the project and the team|
|Involved in the shared comprehension of the team and its commitment||Non||Yes, it is the main strength of Story Points|
|Consistency of the value during the life cycle of the project||Yes||Hard to monitor (inflation of the value of the SP during the project)|
|Measure of the project assessment||Yes||No, since it is not standard|
|Contractualization||Yes||Yes, with some hesitations concerning the consistency of the value of DP during the project lyfe cycle.|
IFPUG MetricViews February 2016:
Dan French, PMP, CFPS, CSM. Agile and Function Points: A Winning Combination: http://www.iceaaonline.com/ready/wp-content/uploads/2016/06/SS03-ppt-Agile-Function-Winning.pdf
Carol Dekkers, CFPS (Fellow), PMP. Counting Function Points for Agile / Iterative Software Development: http://www.ifpug.org/Articles/Dekkers-CountingAgileProjects.pdf
Radenko Corovic. Estimer et mesurer la performance des projets agiles avec les points de fonction : http://rsmtechno.ca/Portals/0/Images/Docs/Estimer%20et%20suivre%20les%20projets%20agiles%20avec%20les%20points%20de%20fonction.pdf