Choosing a Function Unit

The first step in constructing a size estimate is to make sure the Function Unit (the component used to describe/capture system size) is appropriate for your project and for the sizing technique you plan to use for estimation. For new projects, the information in the Function Unit section of the Sizing tab was “inherited” from the configuration source – template or existing project – used when the project was created. Note that the data fields in this section are global ones , meaning they apply to all solutions for the project: current, logged, and Balanced Risk.  Changing the function unit or gearing factor will cause the size for any existing solutions to be recalculated to an equivalent size in base size units (captured using the new function unit and/or gearing factor).  Think of the process as “translating” the current size into your preferred function unit and gearing factor.  Of course, you may also want to edit the size (function unit count) – doing so will generate a new size estimate.

Though in most cases you will not need to change the Function Unit section defaults from your configuration source, it’s still a good idea to understand how they are defined and used. In SLIM tools, project size has two aspects:

•      Function Unit. The function unit is a way of representing the size of the developed software product (newly developed, modified, or reused software).  Examples are:  100,000 logical source statements (lines of code), 2000 function points, 300 stories or 900 story points, 250 requirements, or 28 web pages.  Regardless of the function unit chosen, you will be estimating the number of components that will be constructed or modified, tested, and delivered by the current project. QSM support can help with size estimation.

If you need to add a function unit to the list of available size units, contact your site administrator and ask him or her to add the desired unit to the selection list. If an uploaded Estimation project, template, or SLIM-DataManager database contains function units with no counterpart in the Function Units table, the “new” function unit will be added to the Function Units lookup table.  The Administrator will have the option to validate this function unit and include it in drop-down lists available to other SLIM-Collaborate users and projects.

•      Gearing Factor. The gearing factor normalizes projects with size measured in different sizing units to a common frame of reference: the base size unit.  For example, a project with 50 function points at a gearing factor of 50 converts to 2500 (50 FP * 50 base size units per FP) base size units.   One hundred objects at a gearing factor of 300 is equivalent to 30,000 base size units (100 objects * 300 base size units per object). The base size unit represents the smallest identifiable unit of programming work: it is roughly equivalent to the time and effort it takes to write one line of code.  SLIM-Collaborate provides a default gearing factor for each of the function units but you can replace the default gearing factor with one from your history.

The project size is used during benchmarking and calculation of the PI or other productivity metrics (Size per phase 3 effort unit or phase 3 month). Because all of the major management metrics – schedule, effort or cost, defects, productivity – naturally scale with project size, size data is also used to position projects against similarly sized projects on relevant internal or industry trend lines.