Time-dependent Migration of Citations Through PubMed and OvidSP Subsets: a Study on a Series of Simultaneous PubMed and OvidSP Searches

OBJECTIVE: To quantitatively describe (1) differences between search results derived at consecutive time points with the PubMed and OvidSP literature search interfaces over a five day interval, and (2) the migration of citations through different subsets to estimate the timeliness of OvidSP. METHODS: PubMed-Identifiers (PMIDs) of the following subsets were retrieved from PubMed and OvidSP simultaneously (within 8 h) at 11 days in March and April 2010 including 5 consecutive days: as supplied by publisher, in process, PubMed not MEDLINE, and OLDMEDLINE. Search results were compared for difference and intersection sets. The migration of citations on individual level was determined by comparison of corresponding sets over several days. RESULTS: The "in process" set was stable with about 446,000 - 452,000 citations; a small fraction of up to 3% of the total subsets were in PubMed only and OvidSP only subsets. About 96% of the ca. 10,500 citations in the OvidSP only subset migrated within 2 days out of the "in process" subset. The database of OvidSP is updated within a period of two days.