Figure.

The change of an outcome over time. The time point of the intervention of interest is marked by the dotted line. When there is a trend in the preintervention period that continues into the postintervention period, a simple “before-and-after” comparison of mean values or proportions may misleadingly suggest an effect of the intervention when there is, in fact, none (A). Segmented regression fits 2 separate lines, 1 for the pre- and 1 for the postintervention period. An abrupt change in the outcome after the intervention (B) or a change in the slope (C) suggests a valid association between the intervention and the outcome.