Grey literature is research material that has not been formally published. By its nature, grey literature is hard to locate and the content is of variable quality. Examples of grey literature include: theses, government reports, conference proceedings, research reports, newsletters, article pre-prints etc.
It is recommended that grey literature sources are considered when undertaking a systematic review. Some sources of grey literature are listed on this page, but please note this is not an exhaustive list.
Consider the following to decide if grey literature is of good enough quality to use in your review:
You may find it helpful to use an evaluation tool, such as the AACODS Checklist for appraising grey literature
PsycExtra is a database of grey literature material relating to psychology, behavioural sciences and health. Full text is available for the majority of records - look out for the Ovid Databases PDFs link to access the full text.
PsyArXiv is an open access preprint service for Psychology. It contains working papers, unpublished work and articles under review.
It can be worth looking on relevant organisation or charity websites for grey literature on your topic eg National Autistic Society etc.
Conference proceedings can be a useful source of grey literature on your topic. You can search for conference proceedings in some bibliographic databases, or you can use a Google/Google Scholar to search for relevant conference material.
You may be able to find some grey literature on your topic via a Google/Google Scholar search, although do bear in mind that the information you find by this method will be of variable quality!
Try and find out if there are other researchers working on the same area as you. Sometimes they can point you towards other sources of literature that you haven't found by other means.