Study on Differing Results in Westlaw and Lexis for the Same Searches
Susan Nevelow Mart is the author of a study entitled, "Differences in Search Results Using Westlaw and Lexis: The Continued Need for Redundancy in Legal Research", which is available here: http://lawlib.unc.edu/aals/differences.pdf . Westlaw and Lexis will not necessarily generate identical results for the same search - something which I don't think is common knowledge amongst attorneys.
Westlaw relies mainly on human editors to assign headnotes (drafted by those editors) to its classification system, whereas Lexis uses algorithms based on the text of judicial opinions to classify its headnotes. The study concludes that, West "Key Numbers searches return more relevant results than Either Lexis’s More Like This Headnote or Topic.", but each system contains a number of relevant cases that the other system does not. There is also only a 2 % overlap between the Lexis More Like This Headnote search and the Topic classification search. "Because a major difference between the two systems seems to be the degree of dependence on algorithms for creating classification topics and assigning headnotes to each topic, the role of human editors appears to be a definite advantage in returning relevant results.
Even when the results of Keycite and Shepard's cite check searches were compared it was found that only 33% of the total 'hits' were common to both systems.