Metaphor in Diagrams

Alan Blackwell

Unpublished PhD Thesis, University of Cambridge, September 1998

Note for Readers

As there appears to be continuing interest in the topic of this (now aging) thesis, I would like to draw your attention to more recent publications that may be of more direct relevance to contemporary researchers. In particular, the following paper develops many of the philosophical and design practice issues that were not really resolved in my PhD research as reported in the thesis:
Blackwell, A.F. (2006). The reification of metaphor as a design tool. ACM Transactions on Computer-Human Interaction (TOCHI), 13(4), 490-530.

The particular experimental findings of the thesis itself are also summarised in the following paper:
Blackwell, A.F. (2001). Pictorial representation and metaphor in visual language design. Journal of Visual Languages and Computing, 12(3), 223-252.


Modern computer systems routinely present information to the user as a combination of text and diagrammatic images, described as "graphical user interfaces". Practitioners and researchers in Human-Computer Interaction (HCI) generally believe that the value of these diagrammatic representations is derived from metaphorical reasoning; they communicate abstract information by depicting a physical situation from which the abstractions can be inferred.

This assumption has been prevalent in HCI research for over 20 years, but has seldom been tested experimentally. This thesis analyses the reasons why diagrams are believed to assist with abstract reasoning. It then presents the results of a series of experiments testing the contribution of metaphor to comprehension, problem solving, explanation and memory tasks carried out using a range of different diagrams.

The results indicate that explicit metaphors provide surprisingly little benefit for cognitive tasks using diagrams as an external representation. The benefits are certainly small compared to the effects of general expertise in performing computational tasks. Furthermore, the benefit of metaphor in diagram use is largely restricted to mnemonic assistance. This mnemonic effect appears to be greatest when the user of the diagram constructs his or her own metaphor, rather than being presented with a systematic metaphor of the type recommended for use in HCI.


Some parts of the thesis (those that are principally text) are available as HTML documents. For a printable version, including illustrations, see the instructions below for Acrobat download.

Chapter 1: Introduction
Overview of the thesis
Chapter 2: Diagram and metaphor as tools
Diagrams as tools
Metaphor as a tool
Chapter 3: Metacognition among diagram users
Survey 1: Metacognitive statements in the computer science literature
Survey 2: Professional users of conventional programming languages
Survey 3: Users of a visual programming language
Chapter 4: Diagrammatic metaphor in instruction
Experiment 1: Programming with an implicit pictorial metaphor
Experiment 2: Comparison of systematic and nonsense metaphors
Chapter 5: Diagrams and abstract structure generation
Experiment 3: Visual imagery during planning
Experiment 4: Comparing diagrams to text
Experiment 5: Use of incongruent pictorial nodes
Experiment 6: Other factors affecting diagram planning
Chapter 6: Metaphor for mnemonic diagrams
Experiment 7: Comparison of good / bad / no metaphor
Experiment 8: Explicit metaphor as a mnemonic aid
Experiment 9: Comparison of explicit and implicit metaphor
Chapter 7: Conclusions
Review of experimental findings
Related results
Further investigation
Chapter 8: References


The downloadable version of the thesis was originally split into two halves, archived at the department where I conducted the research. The first half (189 pages) contained the complete text of the thesis, including the bibliography. It is completely self-contained, and most people will probably only want to download this half. The second half consists of appendices containing all the experimental stimuli from the nine experiments described in the thesis. The department has since deleted the thesis from its website, although a paper copy is retained in its library. Any readers who consider this policy to be ill-advised are very welcome to contact me, as I will be more than happy to pass on those opinions to the department.

In the meantime, I have recovered an Acrobat PDF version of the first half only.

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