1. Guide to Functional Decomposition

2. Contents What is functional decomposition?
Why is functional decomposition used?
When is functional decomposition used?
How to perform functional decomposition
A sample functional decomposition – step 1
A sample functional decomposition – step 2
A sample functional decomposition – step 3
What level to decompose functions to?
Further assistance

3. What is functional decomposition?
Decomposition is the process of starting at a high level and dividing entities into smaller and smaller related parts.
Functional decomposition is a business analysis technique for breaking down a “business operation” into functional components.
A Functional Decomposition Diagram (FDD) shows a hierarchical organization of the business functions that comprise the business operation. It does not show the sequence of events.
A FDD is distinct from a process flow diagram (PFD), which shows the sequence of events of a business operation or function.

4. Why is functional decomposition used?
The main purpose of functional decomposition is to break up a large or complex business operation or function into smaller and more manageable chunks. It therefore facilitates understanding of the business operation or function and hence is a useful tool in conducting analysis and design.
Functional decomposition is used in determining the functional requirements of a solution and in defining these in the functional requirements document
A large or complex function is more easily understood when broken down using functional decomposition.
Functional decomposition can be used to break up a large or complex business operation into smaller components, prior to developing process flow diagrams.

5. When is functional decomposition used?
Functional decomposition is most commonly used during the analysis phase of a project to produce functional decomposition diagrams as part of the functional requirements document.
It can also be used during the planning, analysis and design phases of a project to help clarify business operations.

6. How to perform functional decomposition
Organise a meeting with the experts, the people managing and working with the business operation.
Identify and name the business operations to be decomposed
For each business operation, start at the top level and ask “what does this business operation consist of?” Draw the first level components.
Decompose the first level components with their functions and continue to decompose to lower levels until sufficient level of detail is achieved.
Hand draw the initial functional decomposition in front of the expert, getting them to confirm the components.
Ask questions to determine the purpose of each function and record this information.

7. How to perform functional decomposition
Check for completeness:
Is the whole business operation represented?
Are all components shown
Are the connections between the components correct?
Refine as necessary.
Review with the experts:
Do an end to end walk-through of the business operation, checking each function to confirm that it is correct.
Ask if there are any other areas of the business operation that are not already covered.

8. A sample Functional Decomposition Diagram - step 1
What is the business operation? Draw the top level.
Course Administration

9. A sample Functional Decomposition Diagram – step 2
What does this business operation consist of?
Draw the first level boxes.
Course Administration
Course Enrolment
Course Completion
Course Attendance
Note that the first level boxes are deliberately positioned in the example to emphasise that the functional decomposition diagram has no implied sequence of events( ie from left to right) by the relative positions of the components.

10. A sample Functional Decomposition Diagram - step 3
For each of the first level boxes ask:
What does this function consist of?
Draw the next level boxes. Repeat for further levels as required.
Course Administration
Course Enrolment
Course Completion
Course Attendance
Course Application
Course Payment
Course Assessment
Course Certification

11. What level to decompose functions to?
The Functional Decomposition Diagram can have several levels of functions that are broken down into finer gradations.
The number of levels to which the decomposition is performed will depend on the size of the initial function or business operation being analysed, the level of definition required and the lowest level of decomposition that is meaningful.

12. Further assistance For additional supporting guides refer to:
Guide to Process Modelling