title: Methods for estimating confidence intervals in interrupted time series analyses of health interventions.
abstract: OBJECTIVE
  Interrupted time series (ITS) is a strong quasi-experimental research design, which is increasingly applied to estimate the effects of health services and policy interventions. We describe and illustrate two methods for estimating confidence intervals (CIs) around absolute and relative changes in outcomes calculated from segmented regression parameter estimates.
  
  
  METHODS
  We used multivariate delta and bootstrapping methods (BMs) to construct CIs around relative changes in level and trend, and around absolute changes in outcome based on segmented linear regression analyses of time series data corrected for autocorrelated errors.
  
  
  RESULTS
  Using previously published time series data, we estimated CIs around the effect of prescription alerts for interacting medications with warfarin on the rate of prescriptions per 10,000 warfarin users per month. Both the multivariate delta method (MDM) and the BM produced similar results.
  
  
  CONCLUSIONS
  BM is preferred for calculating CIs of relative changes in outcomes of time series studies, because it does not require large sample sizes when parameter estimates are obtained correctly from the model. Caution is needed when sample size is small.
authors.affiliation: Department of Ambulatory Care and Prevention, Harvard Medical School and Harvard Pilgrim Health Care, Boston, MA 02215, USA. fang_zhang@hms.harvard.edu
authors.forename: Fang
authors.initials: F
authors.lastname: Zhang
authors.forename: Anita K
authors.initials: AK
authors.lastname: Wagner
authors.forename: Stephen B
authors.initials: SB
authors.lastname: Soumerai
authors.forename: Dennis
authors.initials: D
authors.lastname: Ross-Degnan
mesh_terms: D000925:Anticoagulants; D016001:Confidence Intervals; D000075202:Contraindications; D006801:Humans; D015233:Models, Statistical; D015999:Multivariate Analysis; D055656:Prescriptions; D012044:Regression Analysis; D012107:Research Design; D012494:Sampling Studies; D014859:Warfarin
chemical_list: D000925:Anticoagulants; D014859:Warfarin