The market concentration for anesthesiology, as measured by the Herfindahl–Hirschman Index (HHI) during the study period. For each region, the HHI was calculated by summing the market shares across all providers in the region. The figure shows the percentage of markets classified as nonconsolidated (HHI, 0 to 1,500), moderately consolidated (HHI, 1,500 to 2,500), and highly consolidated (HHI, 2,500 to 10,000) using Department of Justice criteria (see footnote † in Results).