Grey literature is important because it can:
Using Grey Literature in one way of counterbalance to publication or reporting bias because of the alternative perspectives :
"Many scientifically valid studies, especially those with negative results may never be published. If your review is solely based on published literature, which tends to report positive results, your conclusions are more likely to exaggerate. In a lot of cases that exaggeration can be significant." University of South Australia Grey Literature Libguide
Grey literature can often be difficult to track down. This guide will help you identify useful approaches for tracking down grey literature.
This guide was adapted from Lancaster University, with kind permission.