The purposes of including references in scientific research papers are to give credit to the ideas and findings of others and to direct readers to sources of further information.
Whereas review articles, which pull together and interpret a large body of information, cite a large number of references, a research report cites only immediately relevant references. When deciding which references to include in a research paper, select the most valid, the most available, and the fewest references.
The references generally considered most valid are journal articles, because journal articles undergo a review process before being accepted for publication, although the validity of the review process has yet to be established (Lock, A Difficult Balance). Other valid references include books, Ph.D. theses, and some conferences proceedings (those for which papers are reviewed). References considered less valid include abstracts for meetings, because abstracts do not contain enough information to permit assessment of the work, and some conference proceedings (those for which papers are not reviewed). These less valid references should be used primarily to credit the source of an idea, not to support a conclusion or an argument. Similarly, personal communications and unpublished data or unpublished observations should be used only for such purposes as supporting the results of preliminary studies or citing parallel results in another study population. Because these "references" cannot be obtained and assessed, they do not constitute strong evidence and therefore should not be used to support conclusions or arguments.
The most available references for most readers are journal articles. Books are also generally available. Ph.D. theses and proceedings of meetings take more trouble to find. When Ph.D. work is published in journals, cite the journal article rather than the thesis.
Journal articles that have not yet been published but that have been accepted for publication are referred to as being "in press" (American) or "in the press" (British). These articles can be located fairly easily by searching the appropriate journal beginning about the time the paper in which the "in press" reference is cited was accepted. When citing an "in press" paper, include the title of the journal followed by the words "in press."
Articles that have not yet been accepted are not available and therefore should not be included in the reference list. Even if the journal permits references such as "submitted" and "in preparation" in the reference list, work that has not yet been accepted should not be included in the reference list. Instead, it should be referred to in the text either as a personal communication (for work done by others) or as an unpublished observation (for work done by one or more of the authors). The year of the personal communication or observation should be included. Before citing a personal communication, check with ...