The SEQUAL framework is systems modelling reference model for evaluating the quality of models. The SEQUAL framework, which stands for "semiotic quality framework" is developed by John Krogstie and others since the 1990s. The SEQUAL framework is a so-called "top-down quality framework", which is based on semiotic theory, such as the works of Charles W. Morris. Building on these theory it "defines several quality aspects based on relationships between a model, a body of knowledge, a domain, a modeling language, and the activities of learning, taking action, and modeling". Its usefulness, according to Mendling et al. (2006), was confirmed in a 2002 experiment by Moody et al.
History The basic idea behind the SEQUAL framework is, that "conceptual models can be considered as sets of statements in a language, and therefore can be evaluated in semiotic/linguistic terms". A first semiotic framework for evaluating conceptual models was originally proposed by Lindland et al. in the 1994 article "Understanding quality in conceptual modeling". In its initial version, it considered three quality levels:
syntactic, semantic, and pragmatic quality The framework was later extended, and called the SEQUAL framework by Krogstie et al. in the 1995 article "Defining quality aspects for conceptual models". in the 2002 article "Quality of interactive models" Krogstie & Jørgensen extended the initial framework adding more levels of Stamper's semiotic ladder.
SEQUAL framework topics Modeling is an integral part of many technical fields, including engineering, economics, and software engineering. In this context, a model is a formal representation of an organizational system, such as a business model or a formal description of software in UML.
Model activation Model activation, according to John Krogstie (2006), is the process by which a model affects reality. Model activation involves actors interpreting the model and to some extent adjusting their behaviour accordingly. This process can be:
automated, where a software component interprets the model, manual, where the model guides the actions of human actors, or interactive, where prescribed aspects of the model are automatically interpreted and ambiguous parts are left to the users to resolve.
Sets in the Quality Framework The Quality Framework works with a set of eight items:
A: Actors that develop or have to relate to (parts of) the model. Can be persons or tools. L: What can be expressed in the modeling language M: What is expressed in the model D: What can be expressed about the domain (area of interest) K: The explicit knowledge of the participating persons I: What the persons in the audience interpret the model to say T: What relevant tools interpret the model to say G: The goals of the modeling
Physical quality The three main aspects of physical quality are:
Externalization or the question "Is it possible to externalize knowledge by using the model language?", Internalizability about model persistence and availability, and Basically or the question "Is the model language able to express the model domain?" Externalization is presenting the modeller's concept in some model form for others to make sense of it. Other people can have look on it and can discuss. How other people perceives the model is a matter of internalization. After perceiving the model in their own way they can discuss and change their mind accordingly. To make sense others, it is better to have some model language in common. Physical quality refers to the possibility of externalizing models by using model language that should be available and of course in persistence manner to be internalized by audiences. How available is the model to audience? Availability depends on distributability, especially when members of the audience are geographically dispersed. Then, a model which is an electronically distributable format will be more easily distributed than one which must be printed on paper and sent by ordinary mail or fax. It may also matter exactly what is distributed, e.g. the model in an editable form or merely in an output format. How persistent is the model, how protected is it against loss or damage? This also includes previous versions of the model, if these are relevant. E.g. for a model on disk, the physical quality will be higher if there is a backup copy, or even higher if this backup is on another disk whose failure is independent of the originals. Similarly, for models on paper, the amount and security of backup copies will be essential.
Empirical quality To evaluate empirical quality, the model should be well externalized. Main aspects are:
Ergonomics, readability, layout, and information theory. Basically empirical quality is about the question "Is the model easily readable?". Empirical quality deals with the variety of elements distinguished, error frequencies when being written or read, coding (shapes of boxes) and ergonomics for Computer-Human Interaction for documentation and modeling-tools. Ergonomics is the study of workplace design and the physical and psychological impact it has on workers. This quality is related to readability and layout. There are different factors that have an important impact on visual emphasis like size, solidity, foreground/background differences, colour (red attracts the eye more than other colours), change (blinking or moving symbols attract attention), position and so on. For graph aesthetics there may be different consideration(Battista, 1994, Tamassia, 1988) like angles between edges not be too small, minimize the number of bends along edges, minimize the number of crossings between edges, place nodes with high degree in the centre of the drawing, have symmetry of sons in hierarchies, have uniform density of nodes in the drawing, have verticality of hierarchical structures and so on.
Syntactical quality Syntactic quality is the correspondence between the model M and the language extension L of the language in which the model is written. Three aspects here are:
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