title: Calendar time-specific propensity score analysis for observational data: a case study estimating the effectiveness of inhaled long-acting beta-agonist on asthma exacerbations.
abstract: OBJECTIVE
  Propensity scores (PS) are frequently used in observational studies. PS are usually estimated over the entire study period without consideration of the effect of changing patterns of the included variables over time. This study sought to compare PS estimated using the entire study period (conventional PS) and PS estimated for specific periods (calendar time-specific PS (CTS-PS)), and to determine whether there are differences in estimated treatment effects using these approaches.
  
  
  METHODS
  We conducted a claims data analysis. Asthmatic patients who received an asthma controller during 1997-2008 were included. Exposed patients were those who received an inhaled long-acting beta-2 agonist. Conventional PS used the entire period to estimate a PS for individuals. CTS-PS approach divided the study period into 1-year periods and estimated PS separately for each period. Each individual had two PS. Both PS approaches were used to estimate adjusted hazard ratio (HR) for asthma exacerbations using Cox proportional hazard models.
  
  
  RESULTS
  A total of 288,518 patients with an average age of 11.9 ± 5.8 years were included. The difference between conventional PS and CTS-PS in each period ranged from -0.213 to 0.098. The adjusted HR of conventional PS-matched cohort was 1.20 (95%CI: 1.18-1.22), whereas the estimate for the CTS-PS-matched cohort was 1.24 (95%CI: 1.23-1.37).
  
  
  CONCLUSIONS
  Focusing on a specific year, there was a difference between conventional PS estimated versus CTS-PS for that year. However, there was minimal effect of CTS-PS on the observed treatment effects compared with conventional PS approach.
authors.affiliation: Center of Pharmaceutical Outcomes Research, Department of Pharmacy Practice, Faculty of Pharmaceutical Sciences, Naresuan University, Phitsanulok, Thailand; Center for Pharmacoepidemiology and Pharmacoeconomic Research and Department of Pharmacy Systems, Outcomes and Policy, College of Pharmacy, University of Illinois at Chicago, Chicago, IL, USA.
authors.forename: Piyemeth
authors.initials: P
authors.lastname: Dilokthornsakul
authors.forename: Nathorn
authors.initials: N
authors.lastname: Chaiyakunapruk
authors.forename: Glen T
authors.initials: GT
authors.lastname: Schumock
authors.forename: Todd A
authors.initials: TA
authors.lastname: Lee
mesh_terms: D000280:Administration, Inhalation; D000293:Adolescent; D058666:Adrenergic beta-2 Receptor Agonists / Q000008:administration & dosage / Q000627:therapeutic use*; D000328:Adult; D018927:Anti-Asthmatic Agents / Q000008:administration & dosage / Q000627:therapeutic use*; D001249:Asthma / Q000188:drug therapy* / Q000503:physiopathology; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D016208:Databases, Factual; D005260:Female; D006801:Humans; D008297:Male; D057216:Propensity Score; D016016:Proportional Hazards Models; D012189:Retrospective Studies; D013997:Time Factors; D016896:Treatment Outcome; D055815:Young Adult
chemical_list: D058666:Adrenergic beta-2 Receptor Agonists; D018927:Anti-Asthmatic Agents
keywords: asthma; calendar time-specific propensity score; conventional propensity score; inhaled long-acting beta agonists; pharmacoepidemiology