Top-down and bottom-up are both strategies of
information processing Information processing is the change (processing) of information in any manner detectable by an observer. As such, it is a process that ''describes'' everything that happens (changes) in the universe, from the falling of a rock (a change in pos ...
and knowledge ordering, used in a variety of fields including software, humanistic and scientific theories (see
systemics In the context of systems science and systems philosophy, systemics is an initiative to study systems. It is an attempt at developing logical, mathematical, engineering and philosophical paradigms and frameworks in which physical, technological, bio ...
), and management and organization. In practice, they can be seen as a style of thinking, teaching, or leadership. A top-down approach (also known as ''stepwise design'' and stepwise refinement and in some cases used as a synonym of ''decomposition'') is essentially the breaking down of a system to gain insight into its compositional sub-systems in a reverse engineering fashion. In a top-down approach an overview of the system is 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", which makes it easier to manipulate. However, black boxes may fail to clarify elementary mechanisms or be detailed enough to realistically validate the model. Top down approach starts with the big picture. It breaks down from there into smaller segments. A bottom-up approach is the piecing together of systems to give rise to more complex systems, thus making the original systems sub-systems of the emergent system. Bottom-up processing is a type of
information processing Information processing is the change (processing) of information in any manner detectable by an observer. As such, it is a process that ''describes'' everything that happens (changes) in the universe, from the falling of a rock (a change in pos ...
based on incoming data from the environment to form a
perception Perception (from the Latin ''perceptio'', meaning gathering or receiving) is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All p ...
. From a cognitive psychology perspective, information enters the eyes in one direction (sensory input, or the "bottom"), and is then turned into an image by the brain that can be interpreted and recognized as a perception (output that is "built up" from processing to final cognition). 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, by which 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.

Product design and development

During the design and development of new products, designers and engineers rely on both a bottom-up and top-down approach. The bottom-up approach is being utilized when off-the-shelf or existing components are selected and integrated into the product. An example would include selecting a particular fastener, such as a bolt, and designing the receiving components such that the fastener will fit properly. In a top-down approach, a custom fastener would be designed such that it would fit properly in the receiving components. For perspective, for a product with more restrictive requirements (such as weight, geometry, safety, environment, etc.), such as a space-suit, a more top-down approach is taken and almost everything is custom designed.

Computer science

Software development

:''Part of this section is from the
Perl Design Patterns Book''Perl Design Patterns Book'' is an online textbook about Perl style and design and analysis. The contents are licensed under GNU Free Documentation License. External links * Perl Design Patterns (wiki) Perl Design Patterns Bookin Savannah Catego ...
''. In the
software development process In software engineering, a software development process is the process of dividing software development work into smaller, parallel or sequential steps or subprocesses to improve design, product management, and project management. It is also kn ...
, 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. Top-down approaches are implemented 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 International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the Comput ...
Harlan Mills Harlan D. Mills (May 14, 1919 – January 8, 1996) was Professor of Computer Science at the Florida Institute of Technology and founder of Software Engineering Technology, Inc. of Vero Beach, Florida (since acquired by Q-Labs). Mills' contribu ...
Niklaus Wirth Niklaus Emil Wirth (born 15 February 1934) is a Swiss computer scientist. He has designed several programming languages, including Pascal, and pioneered several classic topics in software engineering. In 1984, he won the Turing Award, generally r ...
. Mills developed
structured programming Structured programming is a programming paradigm aimed at improving the clarity, quality, and development time of a computer program by making extensive use of the structured control flow constructs of selection (if/then/else) and repetition (whi ...
concepts for practical use and tested them in a 1969 project to automate the ''
New York Times ''The New York Times'' (''NYT'' or ''NY Times'') is an American daily newspaper based in New York City with a worldwide readership. Founded in 1851, the ''Times'' has since won 130 Pulitzer Prizes (the most of any newspaper), and has long be ...
'' 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. Among other achievements, Niklaus Wirth, the developer of
Pascal programming language Pascal is an imperative and procedural programming language, designed by Niklaus Wirth as a small, efficient language intended to encourage good programming practices using structured programming and data structuring. It is named in honour of the ...
, wrote the influential paper ''Program Development by Stepwise Refinement''. Since Niklaus Wirth went on to develop languages such as
Modula The Modula programming language is a descendant of the Pascal language. It was developed in Switzerland, at ETH Zurich, in the mid-1970s by Niklaus Wirth, the same person who designed Pascal. The main innovation of Modula over Pascal is a module s ...
Oberon Oberon () is a king of the fairies in medieval and Renaissance literature. He is best known as a character in William Shakespeare's play ''A Midsummer Night's Dream'', in which he is King of the Fairies and spouse of Titania, Queen of the Fairies ...
(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#REDIRECT Software engineering#REDIRECT Software engineering {{Redirect category shell, 1= {{R from other capitalisation ...
{{Redirect category shell, 1= {{R from other capitalisation ...
until the late 1980s, and
object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code: data in the form of fields (often known as ''attributes'' or ''properties''), and code, in the form of procedures (o ...
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 flavor. 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 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. 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 ready for testing. By defining how the application comes together at a high level, lower level work can be self-contained. By defining how the lower level abstractions are expected to integrate into higher level ones, 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, by which the beginnings are small, but eventually grow in complexity and completeness. Object-oriented programming (OOP) is a paradigm that uses "objects" to design applications and computer programs. In mechanical engineering with software programs such as Pro/ENGINEER, Solidworks, and Autodesk Inventor users can design products as pieces not part of the whole and later add those pieces together to form assemblies like building with
Lego Lego ( , ; stylised as LEGO) is a line of plastic construction toys that are manufactured by The Lego Group, a privately held company based in Billund, Denmark. The company's flagship product, Lego, consists of variously coloured interlocking ...
. Engineers call this piece part design. In a bottom-up approach, good intuition is necessary to decide the functionality that is to be provided by the module. If a system is to be built from an existing system, this approach is more suitable as it starts from some existing modules.


Parsing Parsing, syntax analysis, or syntactic analysis is the process of analyzing a string of symbols, either in natural language, computer languages or data structures, conforming to the rules of a formal grammar. The term ''parsing'' comes from Latin ...
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 language#redirect natural language#redirect natural language {{R from other capitalisation ...
{{R from other capitalisation ...
s and
computer languageA computer language is a method of communication with a computer. Types of computer languages include: * Construction language, all forms of communication by which a human can specify an executable problem solution to a computer ** Command language, ...
s, as in a
compiler In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...

. 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 A parse tree or parsing tree or derivation tree or concrete syntax tree is an ordered, rooted tree that represents the syntactic structure of a string according to some context-free grammar. The term ''parse tree'' itself is used primarily in compu ...
structures and then consider whether the known fundamental structures are compatible with the hypothesis. See
Top-down parsingTop-down parsing in computer science is a parsing strategy where one first looks at the highest level of the parse tree and works down the parse tree by using the rewriting rules of a formal grammar. LL parsers are a type of parser that uses a top-do ...
Bottom-up parsingIn computer science, parsing reveals the grammatical structure of linear input text, as a first step in working out its meaning. Bottom-up parsing recognizes the text's lowest-level small details first, before its mid-level structures, and leaving t ...


Top-down and bottom-up are two approaches for the manufacture of products. These terms were first applied to the field of nanotechnology by the
Foresight Institute The Foresight Institute is a Palo Alto, California-based research non-profit that promotes the development of nanotechnology and other emerging technologies. The institute holds conferences on molecular nanotechnology and awards yearly prizes for ...
in 1989 in order to distinguish between molecular manufacturing (to mass-produce large atomically precise objects) and conventional manufacturing (which can mass-produce large objects that are not atomically precise). Bottom-up approaches seek to have smaller (usually Molecule, molecular) components built up into more complex assemblies, while top-down approaches seek to create nanoscale devices by using larger, externally controlled ones to direct their assembly. Certain valuable nanostructures, such as Silicon nanowire#Synthesis, Silicon nanowires, can be fabricated using either approach, with processing methods selected on the basis of targeted applications. 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. Vapor treatment can be regarded as a new top-down secondary approaches to engineer nanostructures. Bottom-up approaches, in contrast, use the chemistry, 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. 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.

Neuroscience and psychology

Image:TheCat.png, An example of top-down processing: Even though the second letter in each word is ambiguous, top-down processing allows for easy disambiguation based on the context. These terms are also employed in neuroscience, cognitive neuroscience and cognitive psychology to discuss the flow of information in processing. Typically Sensory system, sensory input is considered "bottom-up", and Executive functions, higher cognitive processes, which have more information from other sources, are considered "top-down". A bottom-up process is characterized by an absence of higher level direction in sensory processing, whereas a top-down process is characterized by a high level of direction of sensory processing by more cognition, such as goals or targets (Beiderman, 19). According to college teaching notes written by Charles Ramskov, Rock, Neiser, and Gregory claim that top-down approach involves perception that is an active and constructive process. Additionally, it is an approach not directly given by stimulus input, but is the result of stimulus, internal hypotheses, and expectation interactions. According to Theoretical Synthesis, "when a stimulus is presented short and clarity is uncertain that gives a vague stimulus, perception becomes a top-down approach." Conversely, psychology defines bottom-up processing as an approach wherein there is a progression from the individual elements to the whole. According to Ramskov, one proponent of bottom-up approach, Gibson, claims that it is a process that includes visual perception that needs information available from proximal stimulus produced by the distal stimulus. Theoretical Synthesis also claims that bottom-up processing occurs "when a stimulus is presented long and clearly enough." Cognitively speaking, certain cognitive processes, such as fast reactions or quick visual identification, are considered bottom-up processes because they rely primarily on sensory information, whereas processes such as motor system, motor control and Overt attention, directed attention are considered top-down because they are goal directed. Neurologically speaking, some areas of the brain, such as area Visual cortex#Primary visual cortex (V1), V1 mostly have bottom-up connections. Other areas, such as the fusiform gyrus have inputs from higher brain areas and are considered to have top-down influence. The study of Attention, visual attention provides an example. If your attention is drawn to a flower in a field, it may be because the color or shape of the flower are visually salient. 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.

Management and organization

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. A "''top-down''" approach is where an executive decision maker or other top person makes the decisions of how something should be done. 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, when wanting to make an improvement in a hospital, a hospital administrator might decide that a major change (such as implementing a new program) is needed, and then the leader uses a planned approach to drive the changes down to the frontline staff (Stewart, Manges, Ward, 2015). A "''bottom-up''" approach to changes 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. A bottom-up approach can be thought of as "an incremental change approach that represents an emergent process cultivated and upheld primarily by frontline workers" (Stewart, Manges, Ward, 2015, p. 241). 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. Other evidence suggests that there is a third combination approach to change (see Stewart, Manges, Ward, 2015).

Public health

Both top-down and bottom-up approaches exist in public health. There are many examples of top-down programs, often run by governments or large inter-governmental organizations (IGOs); many of these are disease-specific or issue-specific, such as HIV control or Smallpox Eradication of infectious diseases, Eradication. Examples of bottom-up programs include many small NGOs set up to improve local access to healthcare. However, a lot of programs seek to combine both approaches; for instance, guinea worm eradication, a single-disease international program currently run by the Carter Center has involved the training of many local volunteers, boosting bottom-up capacity, as have international programs for hygiene, sanitation, and access to primary health-care.


Often, the École des Beaux-Arts school of design is said to have primarily promoted top-down design because it taught that an architectural design should begin with a parti pris, parti, a basic plan drawing of the overall project. 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 or population dynamics of the ecosystem. The interactions between these top predators and their prey is what influences lower trophic levels. Changes in the top level of trophic levels have an inverse effect on the lower trophic levels. Top-down control can have negative effects on the surrounding ecosystem if there is a drastic change in the number of predators. The classic example is of kelp forest ecosystems. In such ecosystems, sea otters are a keystone species, keystone predator. They prey on Sea urchin, urchins which in turn eat kelp. When otters are removed, urchin populations grow and reduce the kelp forest creating urchin barrens. This reduces the diversity of the ecosystem as a whole and can detrimental effects on all of the other organisms. In other words, such ecosystems are not controlled by productivity of the kelp, but rather, a top predator. One can see the inverse effect that top-down control has in this example; when the population of otters decreased, the population of the urchins increased. Bottom-up control in ecosystems refers to ecosystems in which the nutrient supply, productivity, and type of primary producers (plants and phytoplankton) control the ecosystem structure. 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 the ecological efficiency. 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 common for populations to be influenced by both types of control, and there is still debates going on as to which type of control affects food webs in certain ecosystems.

Philosophy and ethics

Top-down reasoning in ethics is when the reasoner starts from abstract universalisable principles and then reasons down them to particular situations. Bottom-up reasoning occurs when the reasoner starts from intuitive particular situational judgements and then reasons up to principles. Reflective Equilibrium occurs when there is interaction between top-down and bottom-up reasoning until both are in harmony. That is to say, when universalisable abstract principles are reflectively found to be in equilibrium with particular intuitive judgements. The process occurs when cognitive dissonance occurs when reasoners try to resolve top-down with bottom-up reasoning, and adjust one or the other, until they are satisfied they have found the best combinations of principles and situational judgements.

See also

*The Cathedral and the Bazaar * Pseudocode

References cited

* * * * * https://philpapers.org/rec/COHTNO

Citations and notes

Further reading

*Luiz Carlos Bresser-Pereira, José María Maravall, and Adam Przeworski, 1993. Economic reforms in new democracies. Cambridge: Cambridge University Press. . * Dubois, Hans F.W. 2002. Harmonization of the European vaccination policy and the role TQM and reengineering could play. Quality Management in Health Care 10(2): 47–57. * J. A. Estes, M. T. Tinker, T. M. Williams, D. F. Doak "Killer Whale Predation on Sea Otters Linking Oceanic and Nearshore Ecosystems", ''Science'', October 16, 1998: Vol. 282. no. 5388, pp. 473 – 476 * * Galotti, K. (2008). ''Cognitive Psychology: In and out of the laboratory''. USA: Wadsworth. *Goldstein, E.B. (2010). ''Sensation and Perception''. USA: Wadsworth. * *

External links

"Program Development by Stepwise Refinement"
''Communications of the ACM'', Vol. 14, No. 4, April (1971)
Integrated Parallel Bottom-up and Top-down Approach
In Proceedings of the International Emergency Management Society's Fifth Annual Conference (TIEMS 98), May 19–22, Washington DC, USA (1998).
Changing Your Mind: On the Contributions of Top-Down and Bottom-Up Guidance in Visual Search for Feature Singletons
''Journal of Experimental Psychology: Human Perception and Performance'', Vol. 29, No. 2, 483–502, 2003. * K. Eric Drexler and Christine Peterson

Foresight Briefing No. 2, 1989.
Empowering sustained patient safety: the benefits of combining top-down and bottom-up approaches
{{DEFAULTSORT:Top-Down and Bottom-Up Design Dichotomies Information science Neuropsychology Software design Management science Hierarchy