An empirical investigation of the consistency of retrieval through Internet search engines is reported. Thirteen engines are evaluated: AltaVista, EuroFerret, Excite, HotBot, InfoSeek, Lycos, MSN, NorthernLight, Snap, WebCrawler and three national Dutch engines: Ilse, Search.nl and Vindex. The focus is on a characteristics related to size: the degree of consistency to which an engine retrieves documents. Does an engine always present the same relevant documents that are, or were, available in its databases? We observed and identified three types of fluctuations in the result sets of several kinds of searches, many of them significant. These should be taken into account by users who apply an Internet search engine, for instance to retrieve as many relevant documents as possible, or to retrieve a document that was already found in a previous search, or to perform scientometric/bibliometric measurements. The fluctuations should also be considered as a complication of other research on the behaviour and performance of Internet search engines. In conclusion: in view of the increasing importance of the Internet as a publication/communication medium, the fluctuations in the result sets of Internet search engines can no longer be neglected.

Bangui, Bobaya, Kowanyama, Upolu and Zinga are enveloped spherical RNA viruses sensitive to ether and acid pH; morphologically resembling Bunyaviridae, their diameters range from 90 to 100 nm and their bouyant density is 1.17 to 1.18 g/ml in sucrose. St. Floris, a bunya-like virus, serologically related to Rift Valley Fever, has a diameter ranging from 90 to 110 nm. Triniti, Zingilamo, IPYM 120 and virus strain AnB4268 are enveloped spherical RNA viruses sensitive to ether and acid pH, have diameters of 65, 55, 55 and 60 nm and bouyant densities of 1.18, 1.24, 1.20 and 1.18 g/ml in sucrose, respectively; morphologically they resemble Togaviridae. Salanga is a poxvirus measuring 190 X 225 nm. Virus strain AnY1444 is a spherical, non-enveloped RNA virus measuring 85 nm, morphologically resembling Reoviridiae; it is restraint to both ether and acid pH and shows two-peak densities of 1.32 and 1.36 g/ml in caesium chloride. Bangoran, Keuraliba and Yata resemble Rhabdoviridae measuring 60 X 175, 65 X 195 and 60 X 185 nm, respectively.

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