title: Instantaneous preference was a stronger instrumental variable than 3- and 6-month prescribing preference for NSAIDs.
abstract: OBJECTIVE
  Prescriber preference has been used as an instrumental variable (IV) in a prior study of nonselective nonsteroidal anti-inflammatory drugs (NSAIDs) vs. selective cyclooxygenase-2 (COX-2) inhibitors, with preference expressed as the drug constituting the immediately preceding prescription by the same prescriber (instantaneous preference). We sought to compare the correlations between different IV measures with exposure.
  
  
  METHODS
  In an ambulatory electronic medical record database of university-based physicians, we compared correlations with exposure among three measures of prescriber preference: instantaneous preference, and the proportion of that prescriber's prescriptions in the past 3 and 6 months that were for an NSAID.
  
  
  RESULTS
  We identified 37,934 initial NSAID/COX-2 prescriptions. The correlation with exposure was 0.283 (95% confidence interval 0.274-0.292) for instantaneous preference, 0.197 (0.187-0.206) for 3-month preference, and 0.170 (0.160-0.180) for 6-month preference.
  
  
  CONCLUSIONS
  Instantaneous NSAID/COX-2 prescribing preference was most strongly correlated, and therefore the strongest IV. Future research should focus on the robustness of IV methods to violations of underlying assumptions, extension of IV methods to more than two groups, ratio measures of association, second and subsequent prescriptions per person, and time-varying exposures.
authors.affiliation: Center for Clinical Epidemiology & Biostatistics, Department of Biostatistics & Epidemiology, University of Pennsylvania School of Medicine, Philadelphia, PA, USA; Center for Education and Research on Therapeutics, University of Pennsylvania School of Medicine, Philadelphia, PA, USA. Electronic address: hennessy@mail.med.upenn.edu.
authors.forename: Sean
authors.initials: S
authors.lastname: Hennessy
authors.affiliation: Center for Clinical Epidemiology & Biostatistics, Department of Biostatistics & Epidemiology, University of Pennsylvania School of Medicine, Philadelphia, PA, USA; Center for Education and Research on Therapeutics, University of Pennsylvania School of Medicine, Philadelphia, PA, USA.
authors.forename: Charles E
authors.initials: CE
authors.lastname: Leonard
authors.affiliation: Center for Clinical Epidemiology & Biostatistics, Department of Biostatistics & Epidemiology, University of Pennsylvania School of Medicine, Philadelphia, PA, USA; Center for Education and Research on Therapeutics, University of Pennsylvania School of Medicine, Philadelphia, PA, USA.
authors.forename: Cristin M
authors.initials: CM
authors.lastname: Palumbo
authors.affiliation: Center for Clinical Epidemiology & Biostatistics, Department of Biostatistics & Epidemiology, University of Pennsylvania School of Medicine, Philadelphia, PA, USA; Center for Education and Research on Therapeutics, University of Pennsylvania School of Medicine, Philadelphia, PA, USA.
authors.forename: Xiaoli
authors.initials: X
authors.lastname: Shi
authors.affiliation: Center for Clinical Epidemiology & Biostatistics, Department of Biostatistics & Epidemiology, University of Pennsylvania School of Medicine, Philadelphia, PA, USA; Center for Education and Research on Therapeutics, University of Pennsylvania School of Medicine, Philadelphia, PA, USA.
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Ten Have
mesh_terms: D000367:Age Factors; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000627:therapeutic use*; D015986:Confounding Factors, Epidemiologic; D052246:Cyclooxygenase 2 Inhibitors / Q000627:therapeutic use; D011307:Drug Prescriptions / Q000706:statistics & numerical data; D004363:Drug Utilization / Q000706:statistics & numerical data; D004573:Electrolytes; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D008297:Male; D010414:Pennsylvania; D010818:Practice Patterns, Physicians' / Q000706:statistics & numerical data; D012737:Sex Factors; D013997:Time Factors
chemical_list: D000894:Anti-Inflammatory Agents, Non-Steroidal; D052246:Cyclooxygenase 2 Inhibitors; D004573:Electrolytes; C034130:ORALIT