Project Estimation using Function Point Analysis Approach
Function Point Analysis (FPA) is a sizing measure of clear business significance. First made public by Allan Albrecht of IBM in 1979, the FPA technique quantifies the functions contained within software in terms that are meaningful to the software users. The measure relates directly to the business requirements that the software is intended to address. It can therefore be readily applied across a wide range of development environments and throughout the life of a development project, from early requirements definition to full operational use.
The function point measure itself is derived in a number of stages. Using a standardized set of basic criteria, each of the business functions is a numeric index according to its type and complexity. These indices are totaled to give an initial measure of size which is then normalized by incorporating a number of factors relating to the software as a whole. The end result is a single number called the Function Point index which measures the size and complexity of the software product.
In summary, the function point technique provides an objective, comparative measure that assists in the evaluation, planning, management and control of software production.
Introduction to Function Point Analysis
One of the initial design criteria for function points was to provide a mechanism that both software developers and users could utilize to define functional requirements. It was determined that the best way to gain an understanding of the users' needs was to approach their problem from the perspective of how they view the results an automated system produces. Therefore, one of the primary goals of Function Point Analysis is to evaluate a system's capabilities from a user's point of view. To achieve this goal, the analysis is based upon the various ways users interact with computerized systems. From a user's perspective a system assists them in doing their job by providing five (5) basic functions. Two of these address the data requirements of an end user and are referred to as Data Functions. The remaining three address the user's need to access data and are referred to as Transactional Functions.
The Five Components of Function Points
Â¢ Internal Logical Files
Â¢ External Interface Files
Â¢ External Inputs
Â¢ External Outputs
Â¢ External Inquiries
Internal Logical Files - The first data function allows users to utilize data they are responsible for maintaining. For example, a pilot may enter navigational data through a display in the cockpit prior to departure. The data is stored in a file for use and can be modified during the mission. Therefore the pilot is responsible for maintaining the file that contains the navigational information. Logical groupings of data in a system, maintained by an end user, are referred to as Internal Logical Files (ILF).
External Interface Files - The second Data Function a system provides an end user is also related to logical groupings of data. In this case the user is not responsible for maintaining the data. The data resides in another system and is maintained by another user or system. The user of the system being counted requires this data for reference purposes only. For example, it may be necessary for a pilot to reference position data from a satellite or ground-based facility during flight. The pilot does not have the responsibility for updating data at these sites but must reference it during the flight. Groupings of data from another system that are used only for reference purposes are defined as External Interface Files (EIF).
The remaining functions address the user's capability to access the data contained in ILFs and EIFs. This capability includes maintaining, inquiring and outputting of data. These are referred to as Transactional Functions.
External Input - The first Transactional Function allows a user to maintain Internal Logical Files (ILFs) through the ability to add, change and delete the data. For example, a pilot can add, change and delete navigational information prior to and during the mission. In this case the pilot is utilizing a transaction referred to as an External Input (EI). An External Input gives the user the capability to maintain the data in ILF's through adding, changing and deleting its contents.
External Output - The next Transactional Function gives the user the ability to produce outputs. For example a pilot has the ability to separately display ground speed, true air speed and calibrated air speed. The results displayed are derived using data that is maintained and data that is referenced. In function point terminology the resulting display is called an External Output (EO).
External Inquiries - The final capability provided to users through a computerized system addresses the requirement to select and display specific data from files. To accomplish this a user inputs selection information that is used to retrieve data that meets the specific criteria. In this situation there is no manipulation of the data. It is a direct retrieval of information contained on the files. For example if a pilot displays terrain clearance data that was previously set, the resulting output is the direct retrieval of stored information. These transactions are referred to as External Inquiries (EQ).
In addition to the five functional components described above there are two adjustment factors that need to be considered in Function Point Analysis.
Functional Complexity - The first adjustment factor considers the Functional Complexity for each unique function. Functional Complexity is determined based on the combination of data groupings and data elements of a particular function. The number of data elements and unique groupings are counted and compared to a complexity matrix that will rate the function as low, average or high complexity. Each of the five functional components (ILF, EIF, EI, EO and EQ) has its own unique complexity matrix. The following is the complexity matrix for External Outputs.
1-5 DETs 6 - 19 DETs 20+ DETs
0 or 1 FTRs L L A
2 or 3 FTRs L A H
4+ FTRs A H H
L (Low) 4
A (Average) 5
H (High) 7
Using the examples given above and their appropriate complexity matrices, the function point count for these functions would be:
Function Name Function Type Record
Element Type Data
Element Type File Types
data ILF 3 36 n/a 10
data EIF 1 3 n/a 5
data - add EI n/a 36 1 4
data - change EI n/a 36 1 4
data - delete EI n/a 3 1 3
display EO n/a 20 3 7
display EO n/a 20 3 7
speed display EO n/a 20 3 7
display EQ n/a 1 1 3
Total unadjusted count 50 UFPs
All of the functional components are analyzed in this way and added together to derive an Unadjusted Function Point count.
Value Adjustment Factor - The Unadjusted Function Point count is multiplied by the second adjustment factor called the Value Adjustment Factor. This factor considers the system's technical and operational characteristics and is calculated by answering 14 questions. The factors are:
1. Data Communications
The data and control information used in the application are sent or received over communication facilities.
2. Distributed Data Processing
Distributed data or processing functions are a characteristic of the application within the application boundary.
Application performance objectives, stated or approved by the user, in either response or throughput, influence (or will influence) the design, development, installation and support of the application.
4. Heavily Used Configuration
A heavily used operational configuration, requiring special design considerations, is a characteristic of the application.
5. Transaction Rate
The transaction rate is high and influences the design, development, installation and support.
6. On-line Data Entry
On-line data entry and control information functions are provided in the application.
7. End -User Efficiency
The on-line functions provided emphasize a design for end-user efficiency.
8. On-line Update
The application provides on-line update for the internal logical files.
9. Complex Processing
Complex processing is a characteristic of the application.
The application and the code in the application have been specifically designed, developed and supported to be usable in other applications.
11. Installation Ease
Conversion and installation ease are characteristics of the application. A conversion and installation plan and/or conversion tools were provided and tested during the system test phase.
12. Operational Ease
Operational ease is a characteristic of the application. Effective start-up, backup and recovery procedures were provided and tested during the system test phase.
13. Multiple Sites
The application has been specifically designed, developed and supported to be installed at multiple sites for multiple organizations.
14. Facilitate Change
The application has been specifically designed, developed and supported to facilitate change.
Each of these factors is scored based on their influence on the system being counted. The resulting score will increase or decrease the Unadjusted Function Point count by 35%. This calculation provides us with the Adjusted Function Point count.
Function Point Estimation Worksheet
Description Low Medium High Total
Inputs ___ x 3 ___ x 4 ___ x 6 ___
Outputs ___ x 4 ___ x 5 ___ x 7 ___
Queries ___ x 3 ___ x 4 ___ x 6 ___
Files ___ x 7 ___ x 10 ___ x 15 ___
Program Interfaces ___ x 5 ___ x 7 ___ x 10 ___
Total Unadjusted Function Points (TUFP): _______________
(0=no effect on processing complexity; 5=great effect on processing complexity)
Data communications ___
Heavily use configuration ___
Transaction rate ___
End-user efficiency ___
Complex processing ___
Installation ease ___
Multiple sites ___
Distributed functions ___
On-line data entry ___
On-line update ___
Operational ease ___
Processing Complexity (PC): ______
Adjusted Processing Complexity (PCA) = 0.65 + (0.01 * ____________)
Total Adjusted Function Points (TAFP): ___________ * ____________ =