What question does an interrupted time series design answer?

• A. A level or trend change coinciding with the intervention
• B. A comparison of multiple sites
• C. A cross-sectional snapshot after intervention
• D. A randomized control comparison

Answer: (A)

Interrupted time series asks whether the outcome shows a change in its level or its trajectory right at the moment the intervention is introduced. By using many measurements before and after, you can detect an abrupt shift (level change) or a change in the rate of change over time (trend change) that coincides with the intervention. This helps determine if the intervention had an effect beyond what the pre-existing pattern would predict. It’s not about comparing different sites, a single post-intervention snapshot, or randomized control comparisons, which are different study designs.