Table 1 Variables in the Interrupted Time Series equation^a

Y_t

The rate of TBI related ED visits per 100,000 population in a given month (t).

time_t

Represents month values from 1 (January 2012) to 72 (December 2017).

β₀

The intercept or the outcome at the start of the ICD-9-CM period (baseline level at time 0, t = 0, of December 2011)

β₁

The coefficient for the independent variable of time, that starts at 1 for the first month and 72 for the last month for the 72 total months in the six-year period from January 2012 to December 2017

β₂

The coefficient indicating the level of change in the rate of TBI-related ED visits immediately after the transition

ICD10CM

A dummy variable indicating the start of ICD-10-CM (the interruption in the time series) is 0 for each month prior to October 2015, and 1 for each month beginning October 2015 and after.

β₃

The coefficient indicates the change in the slope of the monthly TBI rate after the transition to ICD-10-CM, compared to slope β₁

time_after_ICD10CM

The variable indicates in rank order or sequential order the months at and after the beginning of ICD-10-CM. This variable is 0 prior to ICD-10-CM, 1 for October 2015, 2 for November 2015, 3 for December 2015, etc.

v_t

The error term represents the random variability not explained by the variables in model