Derivation of the seven propensity groups for splenectomy on the basis of data available at the time of diagnosis and the progression indices. The recursive partitioning model shows the variables used to discriminate between subgroups according to the likelihood of the patients' undergoing splenectomy. Hb, hemoglobin at diagnosis in grams per deciliter; Anemia progr., anemia progression index hemoglobin per month. The size of the subgroup relative to the total population and the percentage of patients in that subgroup in whom splenectomy was performed are represented in each circle.