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Estimating the Accuracy of Jury Verdicts

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  • Bruce D. Spencer

Abstract

Average accuracy of jury verdicts for a set of cases can be studied empirically and systematically even when the correct verdict cannot be known. The key is to obtain a second rating of the verdict, for example, the judge's, as in the recent study of criminal cases in the United States by the National Center for State Courts (NCSC). That study, like the famous Kalven‐Zeisel study, showed only modest judge‐jury agreement. Simple estimates of jury accuracy can be developed from the judge‐jury agreement rate; the judge's verdict is not taken as the gold standard. Although the estimates of accuracy are subject to error, under plausible conditions they tend to overestimate the average accuracy of jury verdicts. The jury verdict was estimated to be accurate in no more than 87 percent of the NCSC cases (which, however, should not be regarded as a representative sample with respect to jury accuracy). More refined estimates, including false conviction and false acquittal rates, are developed with models using stronger assumptions. For example, the conditional probability that the jury incorrectly convicts given that the defendant truly was not guilty (a “Type I error”) was estimated at 0.25, with an estimated standard error (s.e.) of 0.07, the conditional probability that a jury incorrectly acquits given that the defendant truly was guilty (“Type II error”) was estimated at 0.14 (s.e. 0.03), and the difference was estimated at 0.12 (s.e. 0.08). The estimated number of defendants in the NCSC cases who truly are not guilty but are convicted does seem to be smaller than the number who truly are guilty but are acquitted. The conditional probability of a wrongful conviction, given that the defendant was convicted, is estimated at 0.10 (s.e. 0.03).

Suggested Citation

  • Bruce D. Spencer, 2007. "Estimating the Accuracy of Jury Verdicts," Journal of Empirical Legal Studies, John Wiley & Sons, vol. 4(2), pages 305-329, July.
  • Handle: RePEc:wly:empleg:v:4:y:2007:i:2:p:305-329
    DOI: 10.1111/j.1740-1461.2007.00090.x
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    Cited by:

    1. Obidzinski, Marie & Oytana, Yves, 2019. "Identity errors and the standard of proof," International Review of Law and Economics, Elsevier, vol. 57(C), pages 73-80.
    2. Theodore Eisenberg & Michael Heise, 2009. "Plaintiphobia in State Courts? An Empirical Study of State Court Trials on Appeal," The Journal of Legal Studies, University of Chicago Press, vol. 38(1), pages 121-155, January.
    3. Kanaya, Shin & Taylor, Luke, 2020. "Type I and Type II Error Probabilities in the Courtroom," MPRA Paper 100217, University Library of Munich, Germany.
    4. Sangjoon Kim & Jaihyun Park & Kwangbai Park & Jin‐Sup Eom, 2013. "Judge‐Jury Agreement in Criminal Cases: The First Three Years of the Korean Jury System," Journal of Empirical Legal Studies, John Wiley & Sons, vol. 10(1), pages 35-53, March.
    5. Geoffrey Jones & Wesley O. Johnson & Timothy E. Hanson & Ronald Christensen, 2010. "Identifiability of Models for Multiple Diagnostic Testing in the Absence of a Gold Standard," Biometrics, The International Biometric Society, vol. 66(3), pages 855-863, September.
    6. Matteo Rizzolli, 2016. "Adjudication: Type-I and Type-II Errors," CERBE Working Papers wpC15, CERBE Center for Relationship Banking and Economics.
    7. Christoph Engel & Andreas Glöckner, 2008. "Can We Trust Intuitive Jurors? An Experimental Analysis," Discussion Paper Series of the Max Planck Institute for Research on Collective Goods 2008_36, Max Planck Institute for Research on Collective Goods.
    8. Bruce D. Spencer, 2012. "When Do Latent Class Models Overstate Accuracy for Diagnostic and Other Classifiers in the Absence of a Gold Standard?," Biometrics, The International Biometric Society, vol. 68(2), pages 559-566, June.

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