In meta-analysis, researchers combine the results of individual studies to arrive at cumulative conclusions. Meta-analysts sometimes include “grey literature” in their evidential base, which includes unpublished studies and studies published outside widely available journals. Because grey literature is a source of data that might not employ peer review, critics have questioned the validity of its data and the results of meta-analyses that include it.
Conn, V.S., Valentine, J.C., Cooper, H.M., & Rantz, M.J. (2003). Grey literature in meta-analysis. Nursing Research, 52(4): 256-261.View the PDF