CitEc is an experimental autonomous citation index, that is, it is a software system which is able to automatically extract references out of the full texts of documents and create links between citing references and cited papers.
With its last update, the CitEc database has reached almost three million references and more than one million citations between documents available in RePEc. This is an important threshold but still is far of being a complete set of citations. There are some limits in the references extracion process:
First, the system needs to have open access to a electronic version of the documents full text. Many journals listed in RePEc have restricted access and therefore are excluded of CitEc unless they grant special access or push the citations to RePEc in other ways. We are working with some publishers that kindly provide us with metadata about references. We try to get on board as many publishers as possible but unfortunately not all of them are willing to collaborate with us at this time. As a result, the data set is still made up mainly of references extracted from working papers. This has the advantage of provide the most updated data about citations since working papers contains the most recent research results.
Second, the URL provided by the RePEc archive maintainer must be correct and must point to the PDF file containing the document full text and not to an intermediate abstract page or similar. Some archives provides this kind of links to force the researchers to pass through their institutional web pages. The system is unable to follow the links to the hidden papers and they are missed in the references extraction process.
The third limit is more technical. In order to extract references, the PDFs files need to be converted into plain ASCII text. This step is key to successfully complete the process, since a good quality text representation of the document makes easier the identification of references. There are a wide variety of PDF files created in different ways and not all of them can be converted.
Finally, the systems does a parsing of the references section, which first needs to be isolated, to identify each reference and split it in its parts: title, author, year, etc. The parsing is done using pattern matching techniques which in some cases are not able to identify the full list of existing references.
As the las update as of December 31, 2007, the CitEc numbers are: 527,357 articles and working papers available in RePEc. Of them, 343,441 cannot be processed by the system due to limitations mentioned in the first two points above, namely:
101,886 have not an electronic representation
216,110 have restricted access
19,174 have not a direct link to the docuent full text
6,271 have wrong url
That leaves an amount of 183,916 documents available to be processed by CitEc. Of them, the process was successfully completed in 134,130 papers, that is the 73% of the available documents. The complete list of sources and the number of processed documents for each series or journal is available here.
All the previous considerations should be taken into account when CitEc data is used for scientific evaluation purposes. We still consider the data to be experimental.
From the point of view of RePEc archive maintainers there are a few basic steps they can take to improve the situation. For example:
- provide direct and correct URLs to the documents full text
- make use of the X-File-Ref to give the system an ASCII version of the references section of a particular document
- help us to lobby the publishers and editors of the restricted journals asking them to send us metadata about references.