Top-down and bottom-up are strategies of information processing and knowledge ordering, mostly involving software, but also other humanistic and scientific theories (see systemics). In practice, they can be seen as a style of thinking and teaching. In many cases top-down is used as a synonym of analysis or decomposition, and bottom-up of synthesis.
A top-down approach is essentially breaking down a system to gain insight into its compositional sub-systems. In a top-down approach an overview of the system is first formulated, specifying but not detailing any first-level subsystems. Each subsystem is then refined in yet greater detail, sometimes in many additional subsystem levels, until the entire specification is reduced to base elements. A top-down model is often specified with the assistance of "black boxes" that make it easier to manipulate. However, black boxes may fail to elucidate elementary mechanisms or be detailed enough to realistically validate the model.
A bottom-up approach is piecing together systems to give rise to grander systems, thus making the original systems sub-systems of the emergent system. In a bottom-up approach the individual base elements of the system are first specified in great detail. These elements are then linked together to form larger subsystems, which then in turn are linked, sometimes in many levels, until a complete top-level system is formed. This strategy often resembles a "seed" model, whereby the beginnings are small but eventually grow in complexity and completeness. However, "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.
In the software development process, the top-down and bottom-up approaches play a key role.
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. 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. Bottom-up emphasizes coding and early testing, which can begin as soon as the first module has been specified. 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. Re-usability of code is one of the main benefits of the bottom-up approach.
Top-down design was promoted in the 1970s by IBM researcher 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. Niklaus Wirth, among other achievements the developer of Pascal programming language, wrote the influential paper Program Development by Stepwise Refinement. Since Niklaus Wirth went on to develop languages such as Modula and Oberon (where one could define a module before knowing about the entire program specification), one can infer that top down programming was not strictly what he promoted. Top-down methods were favored in software engineering until the late 1980s, and object-oriented programming assisted in demonstrating the idea that both aspects of top-down and bottom-up programming could be utilized.
Modern software design approaches usually combine both top-down and bottom-up approaches. 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. Pre-existing modules give designs a bottom-up flavour. Some design approaches also use an approach where a partially-functional system is designed and coded to completion, and this system is then expanded to fulfill all the requirements for the project.
Top-down programming is a programming style, the mainstay of traditional procedural languages, in which design begins by specifying complex pieces and then dividing them into successively smaller pieces. Eventually, the components are specific enough to be coded and the program is written. This is the exact opposite of the bottom-up programming approach which is common in object-oriented languages such as C++ or Java.
The technique for writing a program using top-down methods is to write a main procedure that names all the major functions it will need. Later, the programming team looks at the requirements of each of those functions and the process is repeated. These compartmentalized sub-routines eventually will perform actions so simple they can be easily and concisely coded. When all the various sub-routines have been coded the program is done.
By defining how the application comes together at a high level, lower level work can be self-contained. By defining how the lower level objects are expected to integrate into a higher level object, interfaces become clearly defined.
In a bottom-up approach the individual base elements of the system are first specified in great detail. These elements are then linked together to form larger subsystems, which then in turn are linked, sometimes in many levels, until a complete top-level system is formed. This strategy often resembles a "seed" model, whereby the beginnings are small, but eventually grow in complexity and completeness.
Object-oriented programming (OOP) is a programming paradigm that uses "objects" to design applications and computer programs.
In Mechanical Engineering with software programs such as Pro/ENGINEER and Solidworks users can design products as pieces not part of the whole and later add those pieces together to form assemblies like building LEGOS. Engineers call this piece part design.
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. This method is used in the analysis of both natural languages and computer languages, as in a compiler.
Bottom-up parsing is a strategy for analyzing unknown data relationships that attempts to identify the most fundamental units first, and then to infer higher-order structures from them. Top-down parsers, on the other hand, hypothesize general parse tree structures and then consider whether the known fundamental structures are compatible with the hypothesis. See Top-down parsing and Bottom-up parsing.
Top-down and bottom-up are used as two approaches for assembling nanoscale materials and devices. Bottom-up approaches seek to have smaller (usually molecular) components arrange themselves into more complex assemblies, while top-down approaches seek to create nanoscale devices by using larger, externally-controlled ones to direct their assembly.
The top-down approach often uses the traditional workshop or microfabrication methods where externally-controlled tools are used to cut, mill and shape materials into the desired shape and order. Micropatterning techniques, such as photolithography and inkjet printing belong to this category. Bottom-up approaches, in contrast, use the chemical properties of single molecules to cause single-molecule components to automatically arrange themselves into some useful conformation. These approaches utilize the concepts of molecular self-assembly and/or molecular recognition. See also Supramolecular chemistry.
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.
These terms are also employed in neuroscience and psychology. The study of visual attention provides an example. If your attention is drawn to a flower in a field, it may be simply that the flower is more visually salient than the surrounding field. The information that caused you to attend to the flower came to you in a bottom-up fashion — your attention was not contingent upon knowledge of the flower; the outside stimulus was sufficient on its own.
Contrast this situation with one in which you are looking for a flower. You have a representation of what you are looking for. When you see the object you are looking for, it is salient. This is an example of the use of top-down information.
In cognitive terms, two thinking approaches are distinguished. "Top down" (or "big chunk") is stereotypically the visionary, or the person who sees the larger picture and overview. Such people focus on the big picture and from that derive the details to support it. "Bottom up" (or "small chunk") cognition is akin to focusing on the detail primarily, rather than the landscape. The expression "seeing the wood for the trees" references the two styles of cognition.
A "top down" approach is one where an executive, decision maker, or other person or body makes a decision. This approach is disseminated under their authority to lower levels in the hierarchy, who are, to a greater or lesser extent, bound by them. For example, a structure in which decisions either are approved by a manager, or approved by his authorised representatives based on the manager's prior guidelines, is top-down management.
A "bottom up" approach is one that works from the grassroots — from a large number of people working together, causing a decision to arise from their joint involvement. A decision by a number of activists, students, or victims of some incident to take action is a "bottom-up" decision.
Positive aspects of top-down approaches include their efficiency and superb overview of higher levels. Also, external effects can be internalized. On the negative side, if reforms are perceived to be imposed ‘from above’, it can be difficult for lower levels to accept them (e.g. Bresser Pereira, Maravall, and Przeworski 1993). Evidence suggests this to be true regardless of the content of reforms (e.g. Dubois 2002). A bottom-up approach allows for more experimentation and a better feeling for what is needed at the bottom.
In bottom-up organized organizations, e.g. ministries and their subordinate entities, decisions are prepared by experts in their fields, which define, out of their expertise, the policy they deem necessary. If they cannot agree, even on a compromise, they escalate the problem to the next higher hierarchy level, where a decision would be sought. Finally, the highest common principal might have to take the decision. Information is in the debt of the inferior to the superior, which means that the inferior owes information to the superior. In the effect, as soon as inferiors agree, the head of the organization only provides his “face″ for the decision which his inferiors have agreed upon.
Amongst several countries, the German political system provides one of the purest forms of a bottom-up approach. The German Federal Act on the Public Service provides that any inferior has to consult and support any superiors, that he or she – only – has to follow “general guidelines" of the superiors, and that he or she would have to be fully responsible for any own act in office, and would have to follow a specific, formal complaint procedure if in doubt of the legality of an order . Frequently, German politicians had to leave office on the allegation that they took wrong decisions because of their resistance to inferior experts' opinions (this comonly being called to be “beratungsresistent", or resistant to consultation, in German). The historical foundation of this approach lies with the fact that, in the 19th century, many politicians used to be noblemen without appropriate education, who more and more became forced to rely on consultation of educated experts, which (in particular after the Prussian reforms of Stein and Hardenberg) enjoyed the status of financially and personally independent, indismissable, and neutral experts as Beamte (public servants under public law).
A similar approach can be found in British police laws, where entitlements of police constables are vested in the constable in person and not in the police as an administrative agency, this leading to the single constable being fully responsible for his or her own acts in office, in particular their legality. The experience of two dictatorships in the country and, after the end of such regimes, emerging calls for the legal responsibility of the “aidees of the aidees" (Helfershelfer) of such regimes also furnished calls for the principle of personal responsibility of any expert for any decision made, this leading to a strengthening of the bottom-up approach, which requires maximum responsibility of the superiors.
In the opposite, the French administration is based on a top-bottom approach, where regular public servants enjoy no other task than simply to execute decisions made by their superiors. As those superiors also require consultation, this consultation is provided by members of a cabinet, which is distinctive from the regular minstry staff in terms of staff and organization. Those members who are not members of the cabinet are not entitled to make any suggestions or to take any decisions of political dimension.
The advantage of the bottom-up approach is the great level of expertise provided, combined with the motivating experience of any member of the administration to be responsible and finally the independent “engine" of progress in that field of personal responsibility. A disadvantage is the lack of democratic control and transparency, this leading, from a democratic viewpoint, to the deferment of actual power of policy-making to faceless, if even unknown, public servants. Even the fact that certain politicians might “provide their face" to the actual decisions of their inferiors might not mitigate this effect, but rather strong parliamentary rights of control and influence in legislative procedures (as they do exist in the example of Germany).
The advantage of the top-bottom principle is that political and administrative responsibilities are clearly distinguished from each other, and that responsibility for political failures can be clearly identified with the relevant office holder. Disadvantages are that the system triggers demotivation of inferiors, who know that their ideas to innovative approaches might not be welcome just because of their position, and that the decision-makers cannot make use of the full range of expertise which their inferiors will have collected.
Administrations in dictatorships traditionally work according to a strict top-down approach. As civil servants below the level of the political leadership are discouraged from making suggestions, they use to suffer from the lack of expertise which could be provided by the inferiors, which regularly leads to a breakdown of the system after an few decades. Modern dictatorships, which the People's Republic of China forms an example of, therefore prefer to define a framework of permissible, or even encouraged, criticism and self-determination by inferiors, which would not affect the major state doctrine, but allows the use of professional and expertise-driven knowledge and the use of it for the decision-making persons in office.
By contrast, the Bauhaus focused on bottom-up design. This method manifested itself in the study of translating small-scale organizational systems to a larger, more architectural scale (as with the woodpanel carving and furniture design).
In ecology, top down control refers to when a top predator controls the structure/population dynamics of the ecosystem. The classic example is of kelp forest ecosystems. In such ecosystems, sea otters are a keystone predator. They prey on urchins which in turn eat kelp. When otters are removed, urchin populations grow and reduce the kelp forest creating urchin barrens. In other words, such ecosystems are not controlled by productivity of the kelp but rather a top predator.
Bottom up control in ecosystems refers to ecosystems in which the nutrient supply and productivity and type of primary producers (plants and phytoplankton) control the ecosystem structure. An example would be how plankton populations are controlled by the availability of nutrients. Plankton populations tend to be higher and more complex in areas where upwelling brings nutrients to the surface.
There are many different examples of these concepts. It is not uncommon for populations to be influenced by both types of control.