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Visualisation of historical events using Lexis pencils
A very common data type in social research is the categorical in which individual cases are coded by some attribute (gender, place of residence, work status, etc.). Sometimes such a variable might be an attempt to locate the individual on some underlying scale that can be assumed continuous such as "social class", but more often than not the categories are strictly nominal and many of arithmetic operations do no apply to them.
Methods for the statistical analysis of such data have been developed for many years and are nowadays widely used, but methods for their visualization available in most packages are primitive. Typically these may consist of a bar chart or "pie" diagram (often misused) and not much else, and the difficulty of visualization is compounded if the time change in such categorical variables also needs to be visualized.
Brian Francis and John Pritchard's Case Study presents the Lexis pencil as a way of visualizing a moderate number of individual time changes in a number of categorical variables. The technique and its development is of interest for its own sake, but also by the way it illustrates three more general points of interest:
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