Res Publica: Law and Social Policy Blog and Forum

Did the Supreme Court Appreciate the Statistical Evidence Pertaining to “Commonality” in Wal-Mart v. Dukes?


By Joseph L. Gastwirth, Weiwen Miao, and Efstathia Bura*


Under Rule 23(a)(2), a party seeking class certification must show there is a question of law or fact common to the class. In support of their claim of class-wide gender-based pay disparities, the plaintiffs in Wal-Mart v. Dukes submitted a regression analysis of salary data for each region. 131 S.Ct. 2541 (2011). Their regression accounted for seniority, weeks worked, job held, performance rating, the particular store where the employee worked, and their gender. Females were found to be paid significantly lower than males in almost all regions. The defendant’s expert analyzed the data for each store, separating out employees of grocery departments. They found that, even though females in about three-fourths of the stores were paid less than their male counterparts, the pay disparity was statistically significant in only 10% of the stores. The majority opinion (id. at 2555) indicates that information about disparities at the national and regional level cannot establish the existence of disparities at individual stores, as a regional disparity may be attributable to a small subset of stores. The dissent (id. slip op. at 6, n.5) states that plaintiffs’ regression showed there were disparities within stores. Unfortunately, neither opinion is statistically correct.

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