This was a nice study, and shows a very easy model to adapt to other research problems. The authors were concerned that many reports of randomized clinical trials (RCTs) seemed to miss out on citing previous RCTs on the same topic. So they dug out a bunch of meta-analysis and looked at the bibliographies of the individual RCTs in those trials. The meta-analysis gives you a reasonably comprehensive history of the RCTs done on a particular topic, and you would think that any RCTs in the meta-analysis should have cited any other RCT in that same meta-analysis that preceded it by at least one year. But that happened very rarely.
Robinson KA, Goodman SN. Systematic Examination of the Citation of Prior Research in Reports of Randomized, Controlled Trials. Ann Intern Med. 2011;154(1):50-55. DOI: 10.7326/0003-4819-154-1-201101040-00007. Available in html format.