Concurrent Task Trees (SE)

ConcurTaskTrees (CCT) is a notation for task-model specifications that has been developed by (Paterno et al. 1997; Paternò 2000b; Mori et al. 2002) to overcome limitations of notations previously used to design interactive applications. Its main purpose is to be an easy-to-use notation that can support the design of real industrial applications, which usually means applications with medium-large dimensions. The CTT notation is supported by the ConcurTaskTrees Environment (CTTE), which is a sophisticated application widely appreciated in the research community (see Fig-ure 44).

The motives for CTT come from the limitations of text-based notations (see Ta-ble 41), such as GOMS (Goals, Operators, Methods, Selection Rules) (Dix et al.

2003) and the User Action Notation (UAN) developed by (Hix & Hartson 1993).

Composition

Affinity

A notation intended to easy model-based UI design

GOMS

at a low level only. With UAN, it is possible to describe the temporal relationships between asynchronous tasks. UAN is a table-based notation indicating user actions, system feedback, and changes in the state of the application. However, the lack of UAN tool support has prevented a wider spread of the approach (Mori et al. 2002).

Figure 44: The ConcurTaskTrees Environment (Paternò 2008a)

In contrast, CTT comes with a graphical syntax, a rich set of (temporal) operators for modelling asynchronous and synchronous tasks and the hierarchical structure of tasks. In all, the main features of ConcurTaskTrees are (Mori et al. 2002; Paternò 2008b):

 Hierarchical structure: the hierarchical structure helps to decompose a task into smaller problems, while still maintaining the relationships between the smaller parts of the solution. The hierarchical structure provides a large range of granularity allowing large and small task structures to be reused, and it enables reusable task structures to be defined at both a low and a high semantic level.

 Graphical syntax: a graphical syntax is often (not always) easier to interpret; in this case it should reflect the logical structure, so it should have a tree-like form.

 Concurrent notation: operators for temporal ordering are used to link subtasks at the same abstraction level. This sort of aspect is usually implicit, expressed informally in the outputs of task analysis. Making the analyst use these opera-tors is a substantial change to normal practice. The reason for this innovation is that after an informal task analysis designers must express clearly the logical temporal relationships. This is because such ordering should be taken into ac-count in the UI implementation to allow the user to perform at any time the tasks that should be active from a semantic point of view.

 Focus on activities: this allows designers to concentrate on the most relevant aspects when designing interactive applications that encompass both user- and system-related aspects, thus avoiding low-level implementation details that at the design stage would only obscure the important factors.

Due to its characteristics, the CTT‟s notation has been shown to be an expressive and flexible notation able to represent concurrent and interactive activities. It is at the same time a compact representation that is able to provide much information in

CTT extends GOMS and UAN

an intuitive way without requiring excessive efforts from the users of the notation (see Table 41).

Table 41: CTT in brief Primary use Hierarchical task modelling

Modelling technique / form Diagrammatic, semi-formal; textual informal (early descriptions, scenarios) Advantages Visual notation that extends text-based forms; can be combined with informal

de-scriptions; special tool support (CTTE); can be saved to XML format

Disadvantages Rather formal; generated UIs are straightforward and less innovative from today’s point of view; not a standard in SE, IxD or BPM

Related to Use-cases; task maps; scenarios

It has been demonstrated in practice in industry that workers without a back-ground in computer science can apply CTT. If it is difficult to immediately create the model from scratch, CTT provides the possibility of loading an informal textual description of a scenario or a use-case and interactively selecting the information of interest for the modelling work. To develop a task model from an informal textual description, the CTTE (see Figure 45) user will analyze the description of the sce-nario and identify the main tasks that occur in this description. In this way, the de-signer can first identify tasks, then create a logical hierarchical structure, and finally, complete the task model (Mori et al. 2002).

Figure 45: An excerpt of a task model developed with CTTE (Paternò 2000a)

As with other tools, CTTE allows the task model to be saved in XML format.

With a specific DTD format for task models specified by CTT, information can be exchanged with other modelling environments such as rendering systems that are Working with CTT

Rendering UIs from the CTT format

space industry (see Figure 46). But on the whole, CTT contributes some very impor-tant elements for identifying a common denominator in task modelling. The tree no-tation avoids similarities with UML as far as possible. Nevertheless, the nono-tation provides a sophisticated set of shapes and connectors that - especially during the ear-lier stages - can be associated with additional textual descriptions. The rich set of temporal operators associated with the hierarchical structure allows designers to de-scribe flexible behaviours in both individual and multi-user activities, taking into ac-count possible interruptions and the various ways in which such behaviours may evolve (Mori et al. 2002). On the other hand, the means provided for modelling fo-cus on just a small sub-set of required artefacts and the CTT notation is not a real standard in SE or any other discipline. The developers of CTT therefore considered integration with IBM Rational Rose and a use-case add-on in order to obtain more thorough support in modelling.

Figure 46: UI developed with CTT (Paternò 2000a)