Compared to other search services, Westlaw and Westlaw Edge always produce more search output with prefiltering searches compared to executing the exact same searches in postfiltering mode. No other tested search services did that. Those services (Bloomberg Law and Lexis) always posted the same number of results regardless of search mode. So why does prefiltering Westlaw-Westlaw Edge searching produce more hits than postfiltering searching?
Kevin Rothenberg tested Westlaw’s search algorithm for answers after confirming this insight from Susan Nevelow Mart’s The Algorithm as a Human Artifact: Implications for Legal [Re]Search. The black box that is Westlaw defeated Rothenberg’s efforts: “clearly, I do not understand some important aspect of West’s search algorithm,” wrote Rothenberg in Prefiltering vs. Postfiltering: Which is the Best Method for Searching? AALL Spectrum Nov.-Dec. 2018 at 36 [recommended but paywalled].
I wonder if the West Search development team would shed some light on this unique phenomenon. Perhaps CRIV can ask. (I doubt Thomson Reuters would provide an illuminating look inside West Search’s black box.)