Research / BFI Working PaperJul 22, 2019

Testing the Validity of the Single Interrupted Time Series Design

Katherine Baicker, Theodore Svoronos

Given the complex relationships between patients’ demographics, underlying health needs, and outcomes, establishing the causal effects of health policy and delivery interventions on health outcomes is often empirically challenging. The single interrupted time series (SITS) design has become a popular evaluation method in contexts where a randomized controlled trial is not feasible. In this paper, we formalize the structure and assumptions underlying the single ITS design and show that it is significantly more vulnerable to confounding than is often acknowledged and, as a result, can produce misleading results. We illustrate this empirically using the Oregon Health Insurance Experiment, showing that an evaluation using a single interrupted time series design instead of the randomized controlled trial would have produced large and statistically significant results of the wrong sign. We discuss the pitfalls of the SITS design, and suggest circumstances in which it is and is not likely to be reliable.

More Research From These Scholars

BFI Working Paper Mar 25, 2020

Out of the Woodwork: Enrollment Spillovers in the Oregon Health Insurance Experiment

Adam Sacarny, Katherine Baicker, Amy Finkelstein
Topics:  Health care
BFI Working Paper Jan 18, 2023

Achieving Universal Health Insurance Coverage in the United States: Addressing Market Failures or Providing a Social Floor?

Katherine Baicker, Amitabh Chandra, Mark Shepard
Topics:  Health care