Graphical trees: the simultaneous display of risk of fetal death by pregnancy history and maternal age.

Many epidemiological studies consider sequences of events over time where the outcome of interest is determined, at least in part, by similar outcomes that have occurred in the past, e.g. risk of fetal death in successive pregnancies. In this situation, a probability tree can provide useful insights into the relationship between successive events. However, probability trees are usually limited to the display of a single variable, whereas the majority of studies also include other risk factors. Analyses involving other factors, particularly those that consider interactions, can produce an abundance of results that make identification of potentially interesting patterns difficult. A new graphical approach is described for simultaneously presenting the effects of two risk factors where one relates to a sequence of events over time. The graphical tree highlights patterns in the raw data and is therefore proposed for use in exploratory analyses and hypothesis generation. The approach is introduced and illustrated in the context of risk factors for fetal death, and an interaction between pregnancy history and maternal age is explored.