Figure 1: Decision tree for risk stratification of readmission for suicide attempt and self-harm following medical hospitalization.

The figure visually depicts a binary tree stratifying risk of readmission for suicide attempt or self-harm following medical hospitalization among individuals with serious mental illness (blue = low risk, red = high risk). Each path from root to leaf node can be translated into a series of ‘if-then’ rules that can be applied to classify observations. Each leaf node is associated with a decision rule, corresponding to the most frequent class label (i.e. attempt vs no attempt) of the observations belonging to that node. The denominator represents the number of total hospitalizations corresponding to that decision rule, and the numerator represents the number of readmissions for suicide attempt or self-injury in the year following the hospitalization. Elixhauser category diagnoses refers to the number of common disease conditions. In-hospital mortality score refers to the van Walraven score, a standardized method of condensing medical comorbidities into a single numeric score discriminating risk of death in the hospital. Top right diagram visually represents risk stratification, where circle size represents the size of the subgroup and shading represents the risk of suicide attempt or self-harm. An example of a tree path (right-most branch): 15,644 index hospitalizations were followed by an all-cause readmission, of these, 389 index hospitalizations were of individuals with a prior suicide attempt or self-harm, of these, 69 (18%) index hospitalizations were followed by a readmission for suicide attempt or self-harm. SMI = Serious Mental Illness.