I. Introduction

Statistical assessments are prominent in many kinds of legal cases, including antitrust, employment discrimination, toxic torts, and voting rights cases.1 This reference guide describes the elements of statistical reasoning. We hope the explanations will help judges and lawyers to understand statistical terminology, to see the strengths and weaknesses of statistical arguments, and to apply relevant legal doctrine. The guide is organized as follows:

  • Section I provides an overview of the field, discusses the admissibility of statistical studies, and offers some suggestions about procedures that encourage the best use of statistical evidence.
  • Section II addresses data collection and explains why the design of a study is the most important determinant of its quality. This section compares experiments with observational studies and surveys with censuses, indicating when the various kinds of study are likely to provide useful results.
  • Section III discusses the art of summarizing data. This section considers the mean, median, and standard deviation. These are basic descriptive statistics, and most statistical analyses use them as building blocks. This section also discusses patterns in data that are brought out by graphs, percentages, and tables.
  • Section IV describes the logic of statistical inference, emphasizing foundations and disclosing limitations. This section covers estimation, standard errors and confidence intervals, p-values, and hypothesis tests.
  • Section V shows how associations can be described by scatter diagrams, correlation coefficients, and regression lines. Regression is often used to infer causation from association. This section explains the technique, indicating the circumstances under which it and other statistical models are likely to succeed—or fail.
  • An appendix provides some technical details.
  • The glossary defines statistical terms that may be encountered in litigation.

1. See generally Statistical Science in the Courtroom (Joseph L. Gastwirth ed., 2000); Statistics and the Law (Morris H. DeGroot et al. eds., 1986); National Research Council, The Evolving Role of Statistical Assessments as Evidence in the Courts (Stephen E. Fienberg ed., 1989) [hereinafter The Evolving Role of Statistical Assessments as Evidence in the Courts]; Michael O. Finkelstein & Bruce Levin, Statistics for Lawyers (2d ed. 2001); 1 & 2 Joseph L. Gastwirth, Statistical Reasoning in Law and Public Policy (1988); Hans Zeisel & David Kaye, Prove It with Figures: Empirical Methods in Law and Litigation (1997).

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