I know this is the general specification for a single group:
$Y_i$ = $\beta_0$ + $\beta_1$*time + $\beta_2$*post + $\beta_3$*timepost
time is a continuous variable indicating the time (e.g., month) of every observation in the study;
post is an indicator for whether the observation is after the introduction of the intervention; and
timepost is time since the introduction of the intervention (0 if pre-intervention).
What is the correct specification for a segmented regression of interrupted time series data with a non-equivalent comparison group?