5. Visualisation of Life Histories

Data visualisation is the graphic presentation of data to reveal complex information at a glance (Steele and Iliinsky 2010). The challenge is to map data to a visual display that reveals the range of values of variables and relations between variables. Visualisation of data can be an effective introduction to formal statistical modelling. Ages at marriage may be displayed as points in a scatter plot to assess the distribution of ages and to identify outliers. The marriage duration of a person may be displayed as a line connecting age at marriage and current age or age at marriage dissolution. The end point may be marked if the marriage has been dissolved and not marked if the marriage is intact at the end of the observation period. Visualisation of life histories poses particular challenges. The first is conceptual. The life history is a multistage process of development in which stages create a basis for subsequent stages. In this book the life course is conceptualised as sequences of states and sequences of events. In each domain of life, a state and event sequence can be identified. A second challenge is embedding. The life course is embedded in a historical context, and the visualisation should reveal how developmental processes vary in time. That requires at least two time scales: age and calendar time. The Lexis diagram, named after the demographer Wilhelm Lexis (1837–1914), meets that challenge. Each line in a Lexis diagram represents the follow-up of a single individual from entry to exit on two time scales: age and calendar time. The Lexis diagram is widely used and has inspired improved visualisations of life histories. Some of that research is reviewed in the brief historical note in Sect. 5.1. A third challenge is to reveal significant information at a glance. The graph should convey essential information and highlight the unexpected.