Function Point

             A Function Point (FP) is a unit of measurement to express the amount of business functionality, an information system (as a product) provides to a user. FPs measure software size. They are widely accepted as an industry standard for functional sizing.

Internal Logical Files

        Internal Logical File (ILF) is a user identifiable group of logically related data or control information that resides entirely within the application boundary. The primary intent of an ILF is to hold data maintained through one or more elementary processes of the application being counted. An ILF has the inherent meaning that it is internally maintained, it has some logical structure and it is stored in a file. (Refer Figure 1)

External Interface Files

        External Interface File (EIF) is a user identifiable group of logically related data or control information that is used by the application for reference purposes only. The data resides entirely outside the application boundary and is maintained in an ILF by another application. An EIF has the inherent meaning that it is externally maintained, an interface has to be developed to get the data from the file. (Refer Figure 1)

External Inputs

External Input (EI) is a transaction function in which Data goes “into” the application from outside the boundary to inside. This data is coming external to the application.

• Data may come from a data input screen or another application.

• An EI is how an application gets information.

• Data can be either control information or business information.

• Data may be used to maintain one or more Internal Logical Files.

• If the data is control information, it does not have to update an Internal Logical File. (Refer Figure 1)

External Outputs

    External Output (EO) is a transaction function in which data comes “out” of the system. Additionally, an EO may update an ILF. The data creates reports or output files sent to other applications.

External Inquiries

    External Inquiry (EQ) is a transaction function with both input and output components that result in data retrieval.

FP Counting Process involves the following steps −

·  Step 1 − Determine the type of count.

·  Step 2 − Determine the boundary of the count.

·  Step 3 − Identify each Elementary Process (EP) required by the user.

·  Step 4 − Determine the unique FP.

·  Step 5 − Measure data functions using EI, EO & EQ.

·  Step 6 − Measure transactional functions using ILF &EIF.

· Step 7−Calculate functional size (unadjusted function point count).

·  Step 8 − Determine Value Adjustment Factor (VAF).

·  Step 9 − Calculate Adjusted function point count.

    Use the ’14 general characteristics (GSC)  ’ of a system to find the degree of influence of each of them.

Compute Value Adjustment Factor(VAF): Use the following formula to calculate VAF

     VAF = (65 + ∑GSC ) / 100

Use the following formula to calculate Adjusted Function Point Count or Function Point Count 

                         AFP = UFP * VAF

Advantages:
·  It can be easily used in the early stages of project planning.
·  It is independent of the programming language.
·  It can be used to compare different projects even if they use different technologies(database, language etc).

Disadvantages:
·  It is not good for real-time systems and embedded systems.

·  Many cost estimation models like COCOMO uses LOC and hence   FPC must be converted to LOC.

For Example: Compute the function point value for grade calculation of students. Assume that it is an average complexity size project the information domain values are as follows 

Number of inputs=13

Number of outputs=4

Number of inquiries=2

Number of External Interfaces (EIF)=5

Number of Internal logical files (ILF)=2

UFP = 13 *( avg value ) + 4 *( avg value ) + 2 *( avg value ) + 5 *( avg value ) + 2 *( avg value ) 

VAF = (65 + ∑GSC ) / 100

AFP = UFP * VAF