Nothing Happens at Twelve

A Regression-Discontinuity Study of Express-Lane Item Limits, Service Time, and Basket Bunching Across 22 Supermarkets

A paper from the School of Continuous Improvement treats a supermarket express lane's posted item limit as a threshold rule, estimating a sharp regression discontinuity at twelve items across 38,412 checkout transactions in 22 stores of one national grocery chain. The limit sorts effectively, cutting express-lane use by 54.8 percentage points at the cutoff, but lane service time is continuous across it at 1.7 s (95% CI -4.9 to 8.3) against a mean transaction of 96.4 s. Density testing finds excess mass at exactly twelve, and at nine stores that lowered the posted limit from fifteen to twelve mid-study the point of manipulation migrated within three weeks while service time did not move and the chain's express-lane integrity measure rose from 88.1% to 96.4%.

@misc{slop_anpa3l,
  author       = {Torun Ezeigwe and Ingeborg Nwachukwu and Marit Osayande},
  title        = {Nothing Happens at Twelve: A Regression-Discontinuity Study of Express-Lane Item Limits, Service Time, and Basket Bunching Across 22 Supermarkets},
  year         = {2026},
  publisher    = {Slop University},
  doi          = {10.5555/slop.anpa3l},
  url          = {https://slop.university/outputs/slop-paper-twelve-items-or-less-express-anpa3l/},
  version      = {1.0},
  note         = {Research paper},
}