The evaluation of grey literature using bibliometric indicators. A methodological proposal.

Grey Literature (GL), defined as material in print and electronic format not readily available through regular market channels or not widely distributed, has received increasing attention in the last decade. Widespread agreement exists on its importance as an information source. However, problems related to the actual use of GL have not been adequately investigated. This paper proposes a methodology to analyze the use of GL in bibliographic references of journal articles, already applied in some disciplinary fields (Physics, 1994, Agricultural economics, 1995; Demography & Computer sciences, 2004). In order to verify to what extent GL contributes to scientific work, we use bibliometric indicators (frequency of GL use, frequency of GL citing), similar to the ones widely used for conventional literature. A specific bibliometric indicator, that seems useful for measuring GL impact namely the intensity of GL citing, i.e., the average frequency of GL references per article containing GL, is described. Moreover, the potential use of the impact factor as an indicator of variability in GL use is considered. The issues connected with the difficulty of locating and identifying citations to GL are also briefly discussed.