The Bartik instrument is formed by interacting local industry shares and national industry growth rates. We show that the typical use of a Bartik instrument assumes a pooled exposure research design, where the shares measure differential exposure to common shocks, and identification is based on exogeneity of the shares. Next, we show how the Bartik instrument weights each of the exposure designs. Finally, we discuss how to assess the plausibility of the research design. We illustrate our results through two applications: estimating the elasticity of labor supply, and estimating the elasticity of substitution between immigrants and natives. (JEL C51, F14, J15, J22, L60, R23, R32)

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NBER working papers are circulated for discussion and comment purposes. They have not been peer-reviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.

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Standard-Nutzungsbedingungen:Die Dokumente auf EconStor dürfen zu eigenen wissenschaftlichen Zwecken und zum Privatgebrauch gespeichert und kopiert werden.Sie dürfen die Dokumente nicht für öffentliche oder kommerzielle Zwecke vervielfältigen, öffentlich ausstellen, öffentlich zugänglich machen, vertreiben oder anderweitig nutzen.Sofern die Verfasser die Dokumente unter Open-Content-Lizenzen (insbesondere CC-Lizenzen) zur Verfügung gestellt haben sollten, gelten abweichend von diesen Nutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte. This paper presents preliminary findings and is being distributed to economists and other interested readers solely to stimulate discussion and elicit comments. The views expressed in this paper are those of the authors and are not necessarily reflective of views the Federal Reserve Bank of New York or the Federal Reserve System. Any errors or omissions are the responsibility of the authors. Terms of use: Documents in EconStor may MBS Ratings and the Mortgage Credit BoomAdam Ashcraft, Paul and James Vickery Federal Reserve Bank of New York Staff Reports, no. 449 May 2010 JEL classification: G01, G21, G24 AbstractWe study credit ratings on subprime and Alt-A mortgage-backed-securities (MBS) deals issued between 2001 and 2007, the period leading up to the subprime crisis. The fraction of highly rated securities in each deal is decreasing in mortgage credit risk (measured either ex ante or ex post), suggesting that ratings contain useful information for investors. However, we also find evidence of significant time variation in risk-adjusted credit ratings, including a progressive decline in standards around the MBS market peak between the start of 2005 and mid-2007. Conditional on initial ratings, we observe underperformance (high mortgage defaults and losses and large rating downgrades) among deals with observably higher risk mortgages based on a simple ex ante model and deals with a high fraction of opaque lowdocumentation loans. These findings hold over the entire sample period, not just for deal cohorts most affected by the crisis.Key words: credit rating agencies, mortgages, mortgage-backed securities, subprime crisis Critics interpret these facts as evidence of important flaws in the credit rating process, either due to incentive problems associated with the "issuer-pays" rating model, or simply insufficient diligence or competence (e.g. US Senate, 2010;White, 2009; Fons, 2008). 1 In their defense however, rating agencies argue that recent MBS performance primarily reflects a set of large, unexpected shocks, including an unprecedented decline in home prices, and a financial crisis, events which surprised most market participants. CRAs also point to warnings made by them before the crisis about increasing risk amongst subprime MBS, and argue that ratings became accordingly more conservative to reflect this greater risk. Our main unit of analysis is an MBS deal, which is a set of structured bonds linked to a common pool (or pools) of mortgages. Rat...