IDEAS home Printed from https://ideas.repec.org/a/oup/qjecon/v119y2004i1p249-275..html
   My bibliography  Save this article

How Much Should We Trust Differences-In-Differences Estimates?

Author

Listed:
  • Marianne Bertrand
  • Esther Duflo
  • Sendhil Mullainathan

Abstract

Most papers that employ Differences-in-Differences estimation (DD) use many years of data and focus on serially correlated outcomes but ignore that the resulting standard errors are inconsistent. To illustrate the severity of this issue, we randomly generate placebo laws in state-level data on female wages from the Current Population Survey. For each law, we use OLS to compute the DD estimate of its "effect" as well as the standard error of this estimate. These conventional DD standard errors severely understate the standard deviation of the estimators: we find an "effect" significant at the 5 percent level for up to 45 percent of the placebo interventions. We use Monte Carlo simulations to investigate how well existing methods help solve this problem. Econometric corrections that place a specific parametric form on the time-series process do not perform well. Bootstrap (taking into account the autocorrelation of the data) works well when the number of states is large enough. Two corrections based on asymptotic approximation of the variance-covariance matrix work well for moderate numbers of states and one correction that collapses the time series information into a "pre"- and "post"-period and explicitly takes into account the effective sample size works well even for small numbers of states.

Suggested Citation

  • Marianne Bertrand & Esther Duflo & Sendhil Mullainathan, 2004. "How Much Should We Trust Differences-In-Differences Estimates?," The Quarterly Journal of Economics, Oxford University Press, vol. 119(1), pages 249-275.
  • Handle: RePEc:oup:qjecon:v:119:y:2004:i:1:p:249-275.
    as

    Download full text from publisher

    File URL: http://hdl.handle.net/10.1162/003355304772839588
    Download Restriction: Access to full text is restricted to subscribers.
    ---><---

    As the access to this document is restricted, you may want to look for a different version below or search for a different version of it.

    Other versions of this item:

    References listed on IDEAS

    as
    1. MaCurdy, Thomas E., 1982. "The use of time series processes to model the error structure of earnings in a longitudinal data analysis," Journal of Econometrics, Elsevier, vol. 18(1), pages 83-114, January.
    2. Stephen G. Donald & Kevin Lang, 2007. "Inference with Difference-in-Differences and Other Panel Data," The Review of Economics and Statistics, MIT Press, vol. 89(2), pages 221-233, May.
    3. Meyer, Bruce D, 1995. "Natural and Quasi-experiments in Economics," Journal of Business & Economic Statistics, American Statistical Association, vol. 13(2), pages 151-161, April.
    4. Gary Solon, 1983. "Estimating Autocorrelations in Fixed-Effects Models," Working Papers 540, Princeton University, Department of Economics, Industrial Relations Section..
    5. Alberto Abadie, 2005. "Semiparametric Difference-in-Differences Estimators," Review of Economic Studies, Oxford University Press, vol. 72(1), pages 1-19.
    6. Newey, Whitney & West, Kenneth, 2014. "A simple, positive semi-definite, heteroscedasticity and autocorrelation consistent covariance matrix," Applied Econometrics, Russian Presidential Academy of National Economy and Public Administration (RANEPA), vol. 33(1), pages 125-132.
    7. Nickell, Stephen J, 1981. "Biases in Dynamic Models with Fixed Effects," Econometrica, Econometric Society, vol. 49(6), pages 1417-1426, November.
    8. Moulton, Brent R, 1990. "An Illustration of a Pitfall in Estimating the Effects of Aggregate Variables on Micro Unit," The Review of Economics and Statistics, MIT Press, vol. 72(2), pages 334-338, May.
    Full references (including those not matched with items on IDEAS)

    Most related items

    These are the items that most often cite the same works as this one and are cited by the same works as this one.
    1. Mitchell A. Petersen, 2009. "Estimating Standard Errors in Finance Panel Data Sets: Comparing Approaches," Review of Financial Studies, Society for Financial Studies, vol. 22(1), pages 435-480, January.
    2. Marjan Petreski, 2010. "An Overhaul of a Doctrine: Has Inflation Targeting Opened a New Era in Developing-country Peggers?," FIW Working Paper series 057, FIW.
    3. Michele Campolieti & Chris Riddell, 2020. "Does Mediation-Arbitration Reduce Arbitration Rates? Evidence from a Natural Experiment," ILR Review, Cornell University, ILR School, vol. 73(1), pages 211-235, January.
    4. Guido W. Imbens & Jeffrey M. Wooldridge, 2009. "Recent Developments in the Econometrics of Program Evaluation," Journal of Economic Literature, American Economic Association, vol. 47(1), pages 5-86, March.
    5. Susan Athey & Guido W. Imbens, 2006. "Identification and Inference in Nonlinear Difference-in-Differences Models," Econometrica, Econometric Society, vol. 74(2), pages 431-497, March.
    6. Ciani Emanuele & Fisher Paul, 2019. "Dif-in-Dif Estimators of Multiplicative Treatment Effects," Journal of Econometric Methods, De Gruyter, vol. 8(1), pages 1-10, January.
    7. Giuseppe Moscelli & Hugh Gravelle & Luigi Siciliani, 2018. "Effects of Market Structure and Patient Choice on Hospital Quality for Planned Patients," School of Economics Discussion Papers 1118, School of Economics, University of Surrey.
    8. Collier, Paul & Goderis, Benedikt, 2012. "Commodity prices and growth: An empirical investigation," European Economic Review, Elsevier, vol. 56(6), pages 1241-1260.
    9. Nguyen, Hai V., 2013. "Do smoke-free car laws work? Evidence from a quasi-experiment," Journal of Health Economics, Elsevier, vol. 32(1), pages 138-148.
    10. Ryo Okui, 2017. "Misspecification in Dynamic Panel Data Models and Model-Free Inferences," The Japanese Economic Review, Japanese Economic Association, vol. 68(3), pages 283-304, September.
    11. Marjan Petreski, 2013. "Overhaul of a Doctrine," Eastern European Economics, Taylor & Francis Journals, vol. 51(5), pages 46-68, September.
    12. Timothy G. Conley & Christopher R. Taber, 2011. "Inference with "Difference in Differences" with a Small Number of Policy Changes," The Review of Economics and Statistics, MIT Press, vol. 93(1), pages 113-125, February.
    13. Lorenzo Escot & José Fernández-Cornejo & Carlos Poza, 2014. "Fathers’ Use of Childbirth Leave in Spain. The Effects of the 13-Day Paternity Leave," Population Research and Policy Review, Springer;Southern Demographic Association (SDA), vol. 33(3), pages 419-453, June.
    14. Mikosch, Heiner & Sturm, Jan-Egbert, 2012. "Has the EMU reduced wage growth and unemployment? Testing a model of trade union behavior," European Journal of Political Economy, Elsevier, vol. 28(1), pages 27-37.
    15. Adermon, Adrian & Liang, Che-Yuan, 2014. "Piracy and music sales: The effects of an anti-piracy law," Journal of Economic Behavior & Organization, Elsevier, vol. 105(C), pages 90-106.
    16. Jonathan Roth, 2018. "Should We Adjust for the Test for Pre-trends in Difference-in-Difference Designs?," Papers 1804.01208, arXiv.org, revised May 2018.
    17. Georgios Voucharas & Dimitrios Xefteris, 2018. "Women's Political Power and Environmental Outcomes," University of Cyprus Working Papers in Economics 07-2018, University of Cyprus Department of Economics.
    18. Christian Almer & Stefan Boes & Stephan Nuesch, 2013. "How do Housing Prices Adjust After an Environmental Shock? Evidence from a State-Mandated Change in Aircraft Noise Exposure," Department of Economics Working Papers 11/12, University of Bath, Department of Economics.
    19. Hansen, Christian B., 2007. "Generalized least squares inference in panel and multilevel models with serial correlation and fixed effects," Journal of Econometrics, Elsevier, vol. 140(2), pages 670-694, October.
    20. Hansen, Christian B., 2007. "Asymptotic properties of a robust variance matrix estimator for panel data when T is large," Journal of Econometrics, Elsevier, vol. 141(2), pages 597-620, December.

    More about this item

    JEL classification:

    • C10 - Mathematical and Quantitative Methods - - Econometric and Statistical Methods and Methodology: General - - - General
    • C13 - Mathematical and Quantitative Methods - - Econometric and Statistical Methods and Methodology: General - - - Estimation: General

    Lists

    This item is featured on the following reading lists, Wikipedia, or ReplicationWiki pages:
    1. How Much Should We Trust Differences-In-Differences Estimates? (QJE 2004) in ReplicationWiki

    Statistics

    Access and download statistics

    Corrections

    All material on this site has been provided by the respective publishers and authors. You can help correct errors and omissions. When requesting a correction, please mention this item's handle: RePEc:oup:qjecon:v:119:y:2004:i:1:p:249-275.. See general information about how to correct material in RePEc.

    For technical questions regarding this item, or to correct its authors, title, abstract, bibliographic or download information, contact: . General contact details of provider: .

    If you have authored this item and are not yet registered with RePEc, we encourage you to do it here. This allows to link your profile to this item. It also allows you to accept potential citations to this item that we are uncertain about.

    If CitEc recognized a bibliographic reference but did not link an item in RePEc to it, you can help with this form .

    If you know of missing items citing this one, you can help us creating those links by adding the relevant references in the same way as above, for each refering item. If you are a registered author of this item, you may also want to check the "citations" tab in your RePEc Author Service profile, as there may be some citations waiting for confirmation.

    For technical questions regarding this item, or to correct its authors, title, abstract, bibliographic or download information, contact: Oxford University Press The email address of this maintainer does not seem to be valid anymore. Please ask Oxford University Press to update the entry or send us the correct address or Christopher F. Baum (email available below). General contact details of provider: .

    Please note that corrections may take a couple of weeks to filter through the various RePEc services.

    IDEAS is a RePEc service hosted by the Research Division of the Federal Reserve Bank of St. Louis . RePEc uses bibliographic data supplied by the respective publishers.