The WoS database is currently patchy in terms of data on documents published online, especially those without any form of .pdf download. So, for example, bibliographic data on archaeological research articles published in PLoS One is included, but not for Internet Archaeology, or Before Farming. Another issue relates to 'invisible' research publications in archaeology. To the best of my knowledge, the maps presented here do not include any documents from the grey literature of archaeology: documents produced by archaeological contractors that are not publicly accessible. This material is not currently included in the major citation indices and is, therefore, not presented when performing a search. For the time period covered in this study the importance of this grey literature as a potential but rarely used source for research outputs has been recognised (Bradley 2006), but not yet exploited. Citations to such documents will certainly increase as this literature becomes more accessible to researchers (see for example Bradley et al. 2015) as a result of its external curation, cataloguing and presentation by other agencies such as the Archaeological Data Service in the UK and the National Parks Service in the USA.
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