Susan Mart has published an article called "The Relevance of Results Generated by Human Indexing and Computer Algorithms: A Study of West's Headnotes and Key Numbers and LexisNexis's Headnotes and Topics," 102 Law Library Journal 221 (Spring 2010). Here's the abstract:
This article begins the investigation into the different ways results are generated in West's "Custom Digest" and in LexisNexis's "Search by Topic or Headnote" and by KeyCite and Shepard's. The author took ten pairs of matching headnotes from important federal and California cases and reviewed the results sets generated by each classification and citator system for relevance. The differences in the results sets for classification systems and for citator systems raise interesting issues about the efficiency and comprehensiveness of any one system, and the need to adjust research strategies accordingly.