A longitudinal analysis of search engine index size

One of the determining factors of the quality of Web search engines is the size and quality of their index. In addition to its influence on search result quality, the size of the indexed Web can also tell us something about which parts of the WWW are directly accessible to the everyday user. We propose a novel method of estimating the size of a Web search engine's index by extrapolating from document frequencies of words observed in a large static corpus of Web pages. In addition, we provide a unique longitudinal perspective on the size of Google and Bing's indexes over more than eight years, from March 2006 until January 2015. We find that index size estimates of these two search engines tend to vary dramatically over time, with Google generally possessing a larger index than Bing. This result raises doubts about the reliability of previous one-off estimates of the size of the indexable Web. We find that much of this variability can be explained by changes in the indexing and ranking infrastructure of Google and Bing, as well as the distributed nature of Web search engines. This casts further doubt on whether Web search engines can be used reliably for cross-sectional Webometric studies