The criteria for including 'Grey Literature' in a systematic review are:
Relevance: The literature should directly address the research question (Higgins et al., 2019).
Quality: It should meet acceptable standards for methodological rigor (Moher et al., 2015).
Credibility: Sources must be from reputable organizations (e.g., government agencies, academic institutions) (Davis et al., 2008).
Accessibility: The material should be publicly accessible or retrievable (Garg et al., 2019).
Comprehensiveness: It helps reduce publication bias by including non-peer-reviewed sources (Sterne et al., 2016).
References:
Higgins, J. P. T., et al. (2019). Cochrane Handbook for Systematic Reviews of Interventions.
Moher, D., Liberati, A., Tetzlaff, J., Altman, D. G., & The PRISMA Group (2015). Preferred Reporting Items for Systematic Reviews and Meta-Analyses: The PRISMA Statement.
Davis, P., et al. (2008). Grey Literature and Systematic Reviews.
Garg, A., et al. (2019). Systematic Reviews in Health Research.
Sterne, J. A., et al. (2016). Addressing publication bias in systematic reviews.