Triangle plots of three-level explanatory variables

Thalassaemias are genetic blood disorders which cause varying degrees of anaemia. Their geographical distribution suggests a compensating protection against malaria, which kills between 0.5 and 2.5 million people per year in developing countries. Neal Alexander describes a study in Papua New Guinea which estimated this association more directly, and a triangle plot which clarified, for himself and his non-statistician colleagues, the relative risks of malaria for those with none, one or two copies of the relevant haemoglobin mutation.