The term grey literature refers to all information that isn't commercially published. This includes the following types of documents:
It is necessary to search for grey literature when creating systematic reviews and meta-analyses.
The various sources and types of grey literature commonly searched when conducting a systematic review:
Image obtained from: Rose Relevo Slide Presentation from the AHRQ 2011 Annual Conference 9/20/11
As the scope of grey literature is extremely broad, consider the following questions in order to narrow down your search:
Who are your stakeholders?
What kinds of literature are you interested in?
What time period or geographic/geopolitical area is relevant to your search?
Source: Introduction to Grey Literature for Health Sciences by Franklin Sayre
Based on a guide created by the Norris Medical Library, University of Southern California.