An ITS study finds a significant level change but no slope change after a new seat-belt law takes effect. What does each finding tell you?
Think about your answer, then reveal below.
Model answer: The level change indicates an immediate, step-function effect — compliance jumped when the law took effect. The absence of a slope change means there was no gradual trend acceleration afterward; the benefit was front-loaded. This suggests the law worked through immediate behavioral compliance rather than gradual norm change.
Distinguishing level from slope effects is central to ITS interpretation. They imply different causal mechanisms and different long-run projections.
Question 2 Short Answer
A researcher uses ITS to evaluate a statewide gun-control law passed in March. A major mass shooting had occurred in February of the same year. Why is this a problem, and what can the researcher do?
Think about your answer, then reveal below.
Model answer: The February shooting is a simultaneous shock — it may have independently changed behavior or public attention regardless of the law. The researcher should add a control series (a comparable state without the law) to check whether gun-related outcomes changed similarly there, and should conduct a placebo test examining whether the apparent 'effect' shows up in outcomes unrelated to gun control.
Simultaneous shocks are the core validity threat in ITS. The researcher must argue that the timing of the intervention is independent of the confounder, or use design controls to isolate the law's effect.