Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2016 Apr;104(2):143-5.
doi: 10.3163/1536-5050.104.2.009.

Variation in number of hits for complex searches in Google Scholar

Wichor Matthijs Bramer

Variation in number of hits for complex searches in Google Scholar

Wichor Matthijs Bramer. J Med Libr Assoc. 2016 Apr.

Abstract

Objective: Google Scholar is often used to search for medical literature. Numbers of results reported by Google Scholar outperform the numbers reported by traditional databases. How reliable are these numbers? Why are often not all available 1,000 references shown?

Methods: For several complex search strategies used in systematic review projects, the number of citations and the total number of versions were calculated. Several search strategies were followed over a two-year period, registering fluctuations in reported search results.

Results: Changes in numbers of reported search results varied enormously between search strategies and dates. Theories for calculations of the reported and shown number of hits were not proved.

Conclusions: The number of hits reported in Google Scholar is an unreliable measure. Therefore, its repeatability is problematic, at least when equal results are needed.

Keywords: Information Storage and Retrieval; Reproducibility of Results; Review Literature as Topic; Search Engine.

PubMed Disclaimer

Figures

Figure 1
Figure 1
The last results of a Google Scholar search shown on page 93. This page contains only 2 references; pages 94–100 are available but empty.
Figure 2
Figure 2
Changes in number of hits reported excluding citations by Google Scholar
Figure 3
Figure 3
Changes in number of hits reported including citations by Google Scholar
See this image and copyright information in PMC

References

    1. Harzing AW. A longitudinal study of Google Scholar coverage between 2012 and 2013. Scientometrics. 2014;98(1):565–75.
    1. Orduña-Malea E, Ayllón JM, Martín-Martín A, López-Cózar ED. About the size of Google Scholar: playing the numbers 2014. (Available from: http://arxiv.org/abs/1407.6239. [updated 5 Sep 2014; cited 13 Jan 2015].)
    1. Bramer WM, Giustini D, Kramer BM, Anderson P. The comparative recall of Google Scholar versus PubMed in identical searches for biomedical systematic reviews: a review of searches used in systematic reviews. Syst Rev. 2013 Dec 23;2:115. PubMed. PMID: . Pubmed Central PMCID: . - PMC - PubMed
    1. Pitol SP, De Groote SL. Google Scholar versions: do more versions of an article mean greater impact. Libr Hi Tech. 2014;32(4):594–611.
    1. Hjørland B. Classical databases and knowledge organization: a case for Boolean retrieval and human decision-making during searches. J Assoc Inf Sci Tech. 2015;66(8):1559–75.

LinkOut - more resources