Disk Tree Case Study:
Conceptual Models in Interface Design

Tree view showing disk and partition hierarchy

Figure 1, Tree view showing disk and partition hierarchy

The tree view shown in figure 1 is typical of many applications that deal with disks and their contents. In this particular case, the application is a disk copying utility which allows users the choice of copying disks or individual partitions. The tree view control itself is quite common in both desktop and web-based user interfaces. Perhaps its most frequent use is in Windows Explorer to show the contents of disks and folders. While the hierarchical layout can mean a number of things in different circumstances, here it almost always means containment or "aggregation" as it is known in the Unified Modeling Language (UML). Figure 2 shows a class diagram of the relationship. The diamond shape on the line connecting the two boxes and the"multiplicity" values at each end indicate that "a disk contains zero or more partitions".

Class diagram showing disk containing zero or more paritions

Figure 2, Conceptual model communicated by tree view

This may all seem a little simplistic until we come to consider what will happen if we copy a disk as opposed to copying a partition. In figure 1, disk 1 (the second in the diagram) has only one partition. If a disk is just a container for partitions does it matter which we choose to copy in this case? In the most common situation – a single disk with a single partition – the answer is an emphatic "yes". But how is a user to know that?

What this particular approach is failing to communicate is that disks can contain more than partitions. Most systems have at least one disk containing something called a bootstrap which is responsible for loading an operating system. Without it the system will not start (the dreaded "operating system not found" error).

So that users can properly understand the consequence of their actions, they need a conceptual model like this:

Class diagram showing disk consisting of optional bootstrap and partitions

Figure 3, Revised conceptual model including bootstrap

Figure 3 says that a disk consists of zero or one bootstraps plus zero or more partitions. Admittedly, if users do not know what a bootstrap is or whether they need one, we are still a bit stuck. However, they do at least now have the opportunity of identifying a gap in their knowledge rather than having to deal with the frustration of an outcome they cannot begin to understand.

The revised conceptual model requires a minor change to the original tree view. We now want to explain the relationship between the disk and bootstrap as well as allowing users the choice of including the bootstrap when they are copying partitions. The revised design is shown in figure 4.

Revised tree view showing bootstrap as part of disk

Figure 4, Bootstrap added to disk and partition hierarchy

The approach to conceptual modelling illustrated here has a number of benefits:

  • It can be done before, during or after development (although before is the preferred option)
  • User-oriented conceptual models can be compared with conceptual models derived from an application or web site (as in the example of figure 2 versus figure 3)
  • The conceptual model can be used as the "blueprint" for further development. Changes should be limited to those which have only minor impact on the conceptual model or extend it in ways that will be beneficial to users.
  • The modelling notation (UML) is supported by most software design case tools.

We would be happy to help you apply conceptual modelling to your own projects. Please contact us for a discussion without further obligation or with any comments and queries on the approach.

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