Use of Interrupted Time Series Analysis in Evaluating Health Care Quality Improvements

• Published in: Academic pediatrics Vol. 13; no. 6; pp. S38 - S44
• Main Authors: Penfold, Robert B., Zhang, Fang
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.11.2013
• Subjects: Antipsychotic Agents - therapeutic use; Attention Deficit Disorder with Hyperactivity - diagnosis; Attention Deficit Disorder with Hyperactivity - drug therapy; Child, Preschool; Delivery of Health Care - standards; Delivery of Health Care - trends; Female; Health Services Research; Humans; interrupted time series; Interrupted Time Series Analysis; Male; Neonatal and Perinatal Medicine; Pediatrics; Pediatrics - standards; Pediatrics - trends; Quality Assurance, Health Care; Quality Improvement; quasi-experimental; research design; United States; United States; interrupted time series; research design; quasi-experimental; quality improvement
• ISSN: 1876-2859; 1876-2867; 1876-2867
• DOI: 10.1016/j.acap.2013.08.002

Interrupted time series (ITS) analysis is arguably the strongest quasi-experimental research design. ITS is particularly useful when a randomized trial is infeasible or unethical. The approach usually involves constructing a time series of population-level rates for a particular quality improvement focus (eg, rates of attention-deficit/hyperactivity disorder [ADHD] medication initiation) and testing statistically for a change in the outcome rate in the time periods before and time periods after implementation of a policy/program designed to change the outcome. In parallel, investigators often analyze rates of negative outcomes that might be (unintentionally) affected by the policy/program. We discuss why ITS is a useful tool for quality improvement. Strengths of ITS include the ability to control for secular trends in the data (unlike a 2-period before-and-after t test), ability to evaluate outcomes using population-level data, clear graphical presentation of results, ease of conducting stratified analyses, and ability to evaluate both intended and unintended consequences of interventions. Limitations of ITS include the need for a minimum of 8 time periods before and 8 after an intervention to evaluate changes statistically, difficulty in analyzing the independent impact of separate components of a program that are implemented close together in time, and existence of a suitable control population. Investigators must also be careful not to make individual-level inferences when population-level rates are used to evaluate interventions (though ITS can be used with individual-level data). A brief description of ITS is provided, including a fully implemented (but hypothetical) study of the impact of a program to reduce ADHD medication initiation in children younger than 5 years old and insured by Medicaid in Washington State. An example of the database needed to conduct an ITS is provided, as well as SAS code to implement a difference-in-differences model using preschool-age children in California as a comparison group.