Representing uncertainty

One of the weaknesses of most chronographics is that they conceal uncertainty. This was the case with the first modern timeline, the Carte Chronographique of Barbeu-Dubourg, but not the second, Priestley’s Chart of Biography. Priestley wrote:

It is an imperfection which must necessarily attend every chart of this nature, that the time of the death but more especially the time of the birth of eminent men cannot always be found. In this case the compiler must content himself with placing his line as near as he can conjecture from history where his true place was, leaving marks to express the uncertainty there is attending it.

Priestley, Joseph. 1764. Description of a Chart of Biography. J. Johnson, London. p11.

Priestley’s references to uncertainty can be seen in the copy of his Description on Google Books.

It is increasingly recognised that visualisations need to represent uncertainty effectively. 

Scientific data from instruments, numerical models, or interpolation schemes almost invariably contain some degree of error or uncertainty. Display of such scientific data without uncertainty information is incomplete and may lead to erroneous conclusions. Visualization of data with uncertainty information allows more accurate and effective interpretation.

Wittenbrink, C.M., Pang, A.T. and Lodha, S.K. 1996. Glyphs for Visualizing Uncertainty in Vector Fields. IEEE Transactions on Visualization and Computer Graphics 2(3). September 1996.

Priestley, Joseph. 1765. A Chart of Biography. Photo: Stephen Boyd Davis. With the permission of Chetham's Library, Manchester. 

Priestley not only recognised the problem but devised an effective solution within the limits of the available technology. Minor uncertainty is expressed by a dot underneath the end of the lifeline in question. Increasing levels of uncertainty are expressed by replacing the end of the lifeline with between one and three dots. The illustration above includes three examples which are all dots.

Based on a classification of forms of uncertainty suggested by Pham, Streit and Brown (2009), I have itemised some of the uncertainties that might need to be represented in a timeline for today:

• Range of precision: might range from centuries to seconds.
• Dates which are unknown. 
• Dates which are uncertain with various levels of uncertainty.
  
• Dates of inherently fuzzy events such as movements, trends, ‘-isms’ etc.
• Multiple sources: conflicting evidence of dates
• Multiple models: conflicting calendars (Julian, Gregorian, Islamic, Jewish, Buddhist etc).
• Membership of sets prone to views of different experts. Dispute over categories.
• Sets worthy of modelling contested (Priestley acknowledged that his set is dominated by the English and that it would have been different if designed for a different public).
• Need to be able to import existing datasets which may be inadequate in many, perhaps unpredictable, ways.

References

• Pham, B., Streit, A. and Brown, R. Visualisation of Information Uncertainty: Progress and Challenges. In: Zudilova-Seinstra, E., Adriaansen, T. and van Liere, R. (eds.) Trends in Interactive Visualization. Springer, London. 2009. 19-48 
• Priestley, Joseph. 1764. Description of a Chart of Biography. J. Johnson, London. 
• Wittenbrink, C.M., Pang, A.T. and Lodha, S.K. 1996. Glyphs for Visualizing Uncertainty in Vector Fields. IEEE Transactions on Visualization and Computer Graphics 2(3). September 1996.