TRADITIONAL SOFTWARE DEVELOPMENT PROCESSES - Software Design for Six-Sigma: A Roadmap for Excellence - page 49

Top-Down
Bottom-Up

1. A top-down approach is essentially breaking down
a system to gain insight into its compositional
subsystems.
2. Top-down approaches emphasize planning and a
complete understanding of the system. It is inherent
that no coding can begin until a sufficient level of
detail has been reached in the design of at least
some part of the system.
3. A top-down model is often specified with the
assistance of “black boxes” that make it easier to
manipulate.
4. A bottom-up approach is essentially piecing
together systems to give rise to grander systems,
thus making the original systems subsystems of the
emergent system.
5. The reusability of code is one of the main benefits
of the bottom-up approach.
6. Black boxes may fail to elucidate elementary
mechanisms or be detailed enough to validate the
model realistically.
7. The top-down approach is done by attaching the
stubs in place of the module. This, however, delays
testing of the ultimate functional units of a system
until significant design is complete.
8. In a bottom-up approach, the individual base
elements of the system are first specified in great
detail.
9. Bottom-up emphasizes coding and early testing,
which can begin as soon as the first module has
been specified.
10. This approach, however, runs the risk that modules
may be coded without having a clear idea of how
they link to other parts of the system, and that such
linking may not be as easy as first thought.

Although suitable to any kind of project, in the case of
controls projects, it could be done completely top-down
or bottom-up. It is important for control engineers,
therefore, to understand the two approaches and apply
them appropriately in the hybrid approach. Even when
an engineer is working alone, the hybrid approach helps
keep the project organized and the resulting system
usable, maintainable, and extensible.

( Continued )