如何查找网上医学资源.ppt

* * Slide 4 of 31 On of the major problems with the WWW is being compounded by its popularity…it has become the victim of its own success. I remember discovering the first version of the NCSA Mosaic browser back in 1993 when there were only several hundred WWW sites in existence. I thought that it was a pretty cool hypertext tool akin to Apples Hypercard application that I had dabbled in and began encoding some lab documentation in HTML. However, whilst the ease of use and ease of e-publication have been incredibly empowering, it has resulted in a vast amount of dynamic information being placed on-line, dynamic due to a significant turnover rate with documents being added and removed constantly. This has been further compounded by the fact that HTML has no structured indexing built-in. There has been a tremendous effort to externally index WWW documents both in a manual and automated fashion. However, it is rapidly becoming difficult to locate information since manual catalogs are simply not comprehensive enough and automated indexes are not specific enough. * Slide 6 of 31 To understand more abut the indexing dilemma on the WWW, it helps to understand the different levels of indexing that currently occur. This slide shows thedifferent levels of granularity that exist. At the lowest level are WWW sites which might consist of a number of resources, or collections of documents focussed on one particular subject, and the documents are made up of words and other media. WWW indexes and catalogs can focus at any one of these levels. Due to the significant number of WWW sites that exist, automated indexing programs have become very popular. These are referred to as WWW Robots (or bots for short), Spiders, Wanderers, Worms, Crawlers and basically anything that indicates the link traversing mode by which these operate. The program consists of two parts. The indexing engine starts by downloading a page and first extracting all the hyperlinks referenced. It then removes a