Bottom-up and top-down design

Bottom-up and top-down are both strategies of information processing and ordering knowledge, used in a variety of fields including software, humanistic and scientific theories (see systemics), and management and organization.

However, black boxes may fail to clarify elementary mechanisms or be detailed enough to realistically validate the model.

This strategy often resembles a "seed" model, by which the beginnings are small but eventually grow in complexity and completeness.

But "organic strategies" may result in a tangle of elements and subsystems, developed in isolation and subject to local optimization as opposed to meeting a global purpose.

For perspective, for a product with more restrictive requirements (such as weight, geometry, safety, environment), such as a spacesuit, a more top-down approach is taken and almost everything is custom designed.

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.

[2][failed verification] Top-down design was promoted in the 1970s by IBM researchers Harlan Mills and Niklaus Wirth.

Mills developed structured programming concepts for practical use and tested them in a 1969 project to automate the New York Times morgue index.

The engineering and management success of this project led to the spread of the top-down approach through IBM and the rest of the computer industry.

Although an understanding of the complete system is usually considered necessary for good design—leading theoretically to a top-down approach—most software projects attempt to make use of existing code to some degree.

This strategy often resembles a "seed" model, by which the beginnings are small, but eventually grow in complexity and completeness.

Parsing is the process of analyzing an input sequence (such as that read from a file or a keyboard) in order to determine its grammatical structure.

Certain valuable nanostructures, such as Silicon nanowires, can be fabricated using either approach, with processing methods selected on the basis of targeted applications.

[5] Bottom-up approaches, in contrast, use the chemical properties of single molecules to cause single-molecule components to (a) self-organize or self-assemble into some useful conformation, or (b) rely on positional assembly.

Such bottom-up approaches should, broadly speaking, be able to produce devices in parallel and much cheaper than top-down methods but could potentially be overwhelmed as the size and complexity of the desired assembly increases.

Irvin Rock, Neiser, and Richard Gregory claim that the top-down approach involves perception that is an active and constructive process.

[9][better source needed][10] Theoretical synthesis also claims that bottom-up processing occurs "when a stimulus is presented long and clearly enough.

"Top-down" (or "big chunk") is stereotypically the visionary, or the person who sees the larger picture and overview.

[15][clarification needed] Undergraduate (or bachelor) students are taught the basis of top-down bottom-up processing around their third year in the program.

The two main definitions are that bottom-up processing is determined directly by environmental stimuli rather than the individual's knowledge and expectations.

[16] In the fields of management and organization, the terms "top-down" and "bottom-up" are used to describe how decisions are made and/or how change is implemented.

This approach is disseminated under their authority to lower levels in the hierarchy, who are, to a greater or lesser extent, bound by them.

[17] Positive aspects of top-down approaches include their efficiency and superb overview of higher levels;[17] and external effects can be internalized.

There are many examples of top-down programs, often run by governments or large inter-governmental organizations; many of these are disease-or issue-specific, such as HIV control or smallpox eradication.

Examples of bottom-up programs include many small NGOs set up to improve local access to healthcare.

This method manifested itself in the study of translating small-scale organizational systems to a larger, more architectural scale (as with the wood panel carving and furniture design).

Top-down control can have negative effects on the surrounding ecosystem if there is a drastic change in the number of predators.

If there are not enough resources or producers in the ecosystem, there is not enough energy left for the rest of the animals in the food chain because of biomagnification and ecological efficiency.