Since Function Points measures systems from a functional perspective they are independent of technology. Regardless of language, development method, or hardware platform used, the number of function points for a system will remain constant. The only variable is the amount of effort needed to deliver a given set of function points;
The Five Major Components
Since it is common for computer systems to interact with other computer systems, a boundary must be drawn around each system to be measured prior to classifying components. This boundary must be drawn according to the user’s point of view. In short, the boundary indicates the border between the project or application being measured and the external applications or user domain. Once the border has been established, components can be classified, ranked and tallied.
External Inputs (EI) - is an elementary process in which data crosses the boundary from outside to inside. This data may come from a data input screen or another application. The data may be used to maintain one or more internal logical files. The data can be either control information or business information. If the data is control information it does not have to update an internal logical file.
External Outputs (EO) - an elementary process in which derived data passes across the boundary from inside to outside. Additionally, an EO may update an ILF. The data creates reports or output files sent to other applications. These reports and files are created from one or more internal logical files and external interface file.
External Inquiry (EQ) - an elementary process with both input and output components that result in data retrieval from one or more internal logical files and external interface files. The input process does not update any Internal Logical Files, and the output side does not contain derived data.
Internal Logical Files (ILF’s) - a user identifiable group of logically related data that resides entirely within the applications boundary and is maintained through external inputs.
External Interface Files (EIF’s) - a user identifiable group of logically related data that is used for reference purposes only. The data resides entirely outside the application and is maintained by another application. The external interface file is an internal logical file for another application.
All components are rated as Low, Average or High
After the components have been classified as one of the five major components (EI’s, EO’s, EQ’s, ILF’s or EIF’s), a ranking of low, average or high is assigned. For transactions (EI’s, EO’s, EQ’s) the ranking is based upon the number of files updated or referenced (FTR’s) and the number of data element types (DET’s). For both ILF’s and EIF’s files the ranking is based upon record element types (RET’s) and data element types (DET’s). A record element type is a user recognizable subgroup of data elements within an ILF or EIF. A data element type is a unique user recognizable, non recursive, field.
Function Points can be used to size software applications accurately. Sizing is an important component in determining productivity (outputs/inputs).
They can be counted by different people, at different times, to obtain the same measure within a reasonable margin of error.
Each of the following tables assists in the ranking process. For example, an EI that references or updates 2 File Types Referenced (FTR’s) and has 7 data elements would be assigned a ranking of average and associated rating of 4. Where FTR’s are the combined number of Internal Logical Files (ILF’s) referenced or updated and External Interface Files referenced
For both ILF’s and EIF’s the number of record element types and the number of data elements types are used to determine a ranking of low, average or high. A Record Element Type is a user recognizable subgroup of data elements within an ILF or EIF. A Data Element Type (DET) is a unique user recognizable, non recursive field on an ILF or EIF.
Each count is multiplied by the numerical rating shown to determine the rated value. The rated values on each row are summed across the table, giving a total value for each type of component. These totals are then summed across the table, giving a total value for each type of component. These totals are then summoned down to arrive at the Total Number of Unadjusted Function Points (UAF).
The value adjustment factor (VAF) is based on 14 general system characteristics (GSC's).
The degrees of influence range on a scale of zero to five, from no influence to strong influence.
- Data communication
- Distributed functions
- Performance objectives
- Heavily used configuration
- Transaction rate
- On-line data entry
- End-user efficiency
- On-line update
- Complex processing
- Installation ease
- Operational ease
- Multiple sites
- Facilitate change
VAF = 0.65 + ( sum of 14 characteristics ) / 100
The final Function Point Count is obtained by multiplying the VAF times the Unadjusted Function Point (UAF).
FP = UAF * VAF