@TechReport{iza:izadps:dp6339,
            	author={Belzil, Christian and Hansen, Jörgen},
            	title={Characterizing the Instrumental Variable Identifying Assumption as Sample Selection Conditions},
            	year={2012},
	month={Feb},
            	institution={Institute of Labor Economics (IZA)},
            	address={Bonn},
            	type={IZA Discussion Paper},
            	number={6339},
            	url={https://www.iza.org/publications/dp6339},
	abstract={We build on Rosenzweig and Wolpin (2000) and Keane (2010) and show that in order to fulfill the Instrumental variable (IV) identifying moment condition, a policy must be designed so that compliers and non-compliers either have the same average error term, or have an error term ratio equal to their relative share of the population. The former condition (labeled Choice Orthogonality) is essentially a no-selection condition. The latter one, referred to as Weighted Opposite Choices, may be viewed as a distributional (functional form) assumption necessary to match the degree of selectivity between compliers and noncompliers to their relative population proportions. Those conditions form a core of implicit IV assumptions that are present in any empirical applications. They allow the econometrician to gain substantial insight about the validity of a specific instrument, and they illustrate the link between identification and the statistical strength of an instrument. Finally, our characterization may also help designing a policy generating a valid instrument.},
	keywords={instrumental variable methods;implicit assumptions;treatment effects},
            }