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Next: Typologies for a modular Up: A modular structure Previous: The general definition of

    
3.1.3 The composition of modules

The inter-dependence of information units is expressed not only by means of links between modules, but also in the `composition' of modules.3.9 By composition we mean putting together `elementary modules' to form `complex modules'.

An elementary module contains information that cannot be represented in more than one separate module.

Elementary modules are the smallest characterised building blocks of scientific publications, and thereby the prototypical modules. They cannot be further subdivided into smaller modules, because the resulting fragments would not be self-contained as required by definition 3.1.1. Nevertheless, they have an internal structure, which can be made visible in order to enhance the clarity, for instance by sectioning the text.

   
Figure 3.1: In a modular environment, an elementary module is part of a complex module is part of a modular article. Traditionally, a subsection is a part of a section is part of an article

Elementary modules represent information units that readers should be able to locate, retrieve, consult and manipulate as separate entities. When the information units are related, the relations between them can be expressed in links between the modules. However, the coherence of the information represented in a particular set of separate modules can be so strong that links do not suffice to express it. In those cases, readers should be able to deal with the linked elementary modules as a single entity.  Hence, the modular model allows for the composition of strongly related modules into a complex module, as is illustrated in figure 3.1. Unlike an elementary module, a complex module does contain constituent modules satisfying definition 3.1.1: elementary modules or lower-level complex modules that are self-contained in their own right. Being a module itself, a complex module carries a single characterisation and represents a single concept. This is a `complex concept', however, that is related to lower-level `constituent concepts' . 

A complex module consists of a coherent collection of (elementary or complex) constituent modules and the links between them.

The purpose of the distinction of elementary modules and complex modules is to balance on the one hand the self-containedness of the individual elementary modules, and on the other hand the coherence of the information represented in a collection of strongly related modules.

Elementary modules can be seen as atoms: the smallest building blocks. Subdividing the atom, into its protons, neutrons and electrons, eliminates its chemical meaning. A complex module can then be seen as a molecule. A molecule contains atoms, but its chemical properties are uniquely defined and differ completely from the properties of its constituting atoms: the atomic bonds cannot be disregarded without losing the essence of the molecule.

There may be various kinds of relations between the central concept of the complex module and the `constituent concepts' in the constituent modules. In other words, different composition, or `synthesis' mechanisms are possible.  In our model, we shall restrict ourselves to two specific mechanisms: aggregation and generalisation.3.10

Aggregation  is grouping related (albeit possibly dissimilar) objects to form an aggregate object on a higher level. The classical example is that two legs, two arms, a trunk and a head aggregate into a body. An `aggregate complex module' is a complex module consisting of constituent modules that each represent a component of the aggregate concept. The molecule is such an aggregate - in chemical terms: a compound. Therefore, we call this kind of complex module a `compound module'.3.11

A compound module is a complex module that is an aggregate of (elementary or complex) constituent modules.

An example of a compound module is the module representing an experimental set-up that consists of different components, such as a source and a detector, that are separately presented in constituent modules (see section 4.2.2). The compound module can also represent a procedure that consists of different steps.

A compound module has to consist of a necessary minimum of constituent modules to be adequate as an aggregate. That necessary minimum depends on the central concept of the module: which components are gathered into the aggregate of that particular type. In the chemical metaphor, a water molecule must consist of two hydrogen atoms and an oxygen atom. In the case of less rigorously defined concepts, the necessary minimum of an aggregate is less clear cut than the chemical metaphor would suggest. In the canonical example of aggregation, a prototypical body is an aggregate of six components (two arms, two legs, a trunk and a head). A less prototypical body, however, may lack some of these components: one would still call the aggregate a body, if one arm were missing, but one would be less likely to allow for a missing trunk.

The second way of forming a complex module is by putting together similar modules into a generic module by generalisation : ``A generalisation is an abstraction which enables a class of individual objects to be thought of generically as a single named object." [Smith and Smith, 1977, p.107]. All specific kinds of birds, for example, can be grouped under the general term `birds'. Accordingly, a `generalisation complex module' focuses on a general concept, whereas the constituent modules it contains represent specific cases.

In the chemical metaphor, this kind of complex module can be considered a cluster. In chemistry, a cluster is a collection of similar atoms or molecules with the same chemical properties as the individual particles, but different physical properties. A cluster does not change in nature when a particular particle is added or removed. Unlike a compound module, a complex module created by generalisation does not have a particular minimum or maximum set of components. The minimum for a `generalisation complex module' to be useful is to contain at least two constituent modules.

A cluster module is a complex module that focuses on a single concept which is a generalisation of specific concepts dealt with in the (elementary or complex) constituent modules.

An example of a cluster module is the module with results on reactions with alkali atoms in general that contains constituent modules dealing specifically with results with sodium, potassium and lithium atoms.

Complex modules can be created at different levels. A modular article  is a particular type of complex module: it is a a uniquely characterised self-contained representation of an information unit and it is composed of a specified set of related modules. Higher-order complex modules can also be identified, e.g. in a collection of articles. The articles that we have analysed for the development of our modular model are, for instance, aggregated into units corresponding to the different parts of the corpus (see section 5.1.1) and, at a still higher level, aggregated into the corpus that covers the research project as a whole.  



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