Statistical Analysis

Demographic characteristics were summarized with frequencies and percentages, with values from guideline period 1 compared with those from guideline period 2 using χ² tests to identify candidate variables for adjustment. Segmented regression analysis with an interrupted time series (ITS) was conducted with the publication of the 2014 guideline as the event point. This analysis measured rates of change in outcomes over the 2 study periods, defined as the slope associated with use of tests and treatments over each period, and level change, defined as the change in use of tests and treatments between time periods divided by a single event point. Monthly rates of resource use were included in the segmented regression analysis. The model adjusted for the hospital providing service with random intercepts to control for clustering, as well as fixed effects for age, race/ethnicity, and payer. The ITS output is presented as a point estimate with 95% CIs for monthly slopes, change in slope between time periods, and level change between time periods.

All statistical analyses were performed using SAS version 9.4 (SAS Institute) from June through December 2020. Changes were considered statistically significant if the 95% CI did not include 0.