title: A selective review of the first 20 years of instrumental variables models in health-services research and medicine.
abstract: BACKGROUND
  The method of instrumental variables (IV) is useful for estimating causal effects. Intuitively, it exploits exogenous variation in the treatment, sometimes called natural experiments or instruments. This study reviews the literature in health-services research and medical research that applies the method of instrumental variables, documents trends in its use, and offers examples of various types of instruments.
  
  
  METHODS
  A literature search of the PubMed and EconLit research databases for English-language journal articles published after 1990 yielded a total of 522 original research articles. Citations counts for each article were derived from the Web of Science. A selective review was conducted, with articles prioritized based on number of citations, validity and power of the instrument, and type of instrument.
  
  
  RESULTS
  The average annual number of papers in health services research and medical research that apply the method of instrumental variables rose from 1.2 in 1991-1995 to 41.8 in 2006-2010. Commonly-used instruments (natural experiments) in health and medicine are relative distance to a medical care provider offering the treatment and the medical care provider's historic tendency to administer the treatment. Less common but still noteworthy instruments include randomization of treatment for reasons other than research, randomized encouragement to undertake the treatment, day of week of admission as an instrument for waiting time for surgery, and genes as an instrument for whether the respondent has a heritable condition.
  
  
  CONCLUSIONS
  The use of the method of IV has increased dramatically in the past 20 years, and a wide range of instruments have been used. Applications of the method of IV have in several cases upended conventional wisdom that was based on correlations and led to important insights about health and healthcare. Future research should pursue new applications of existing instruments and search for new instruments that are powerful and valid.
authors.affiliation: a Department of Policy Analysis and Management , Cornell University , Ithaca , NY , USA , and School of Economics, University of Sydney , Sydney , Australia.
authors.forename: John
authors.initials: J
authors.lastname: Cawley
mesh_terms: D005838:Genotype; D006302:Health Services Research / Q000379:methods*; D006801:Humans; D015233:Models, Statistical; D010818:Practice Patterns, Physicians'; D011897:Random Allocation; D012107:Research Design; D013997:Time Factors; D014195:Travel
keywords: Econometrics; Health; Instrumental variables; Methods