Guide to Functional Decomposition

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Guide toFunctional Decomposition
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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

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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 organisation 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.
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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.
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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.
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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.

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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.

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A sample Functional Decomposition Diagram - step 1
What is the business operation? Draw the top
level.
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A sample Functional Decomposition Diagram step 2
What does this business operation consist of?
Draw the first level boxes.
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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.
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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.
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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.
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Further assistance
For additional supporting guides refer to

• Guide to Process Modelling