Fig. 1

Multilevel structure used in the models. Most models included the four levels Episode, Treating hospital, Local Health District (LHD) of the treating hospital, and Statistical Local Area (SLA) of residence. Two procedures (spinal fusion, endovascular repair of abdominal aortic aneurysm) occurred at only one hospital per LHD, so the LHD level was excluded in modelling those procedures. Arrows show a nested relationship (each episode occurs at only one hospital; each hospital belongs to only one LHD; each patient lives in only one SLA). Cross-classification occurs between SLA and hospital and between SLA and LHD, meaning patients from the same SLA may be treated in different hospitals and LHDs, while the same hospital may treat patients from different SLAs