Metrics such as citation counts and Journal Impact Factors should be used with great care.
As is delineated in The Metric Tide (Wilsdon, 2015) "quantitative evaluation should support, but not supplant, qualitative, expert assessment" .
In particular please note that:
Anyone using citation counts and researcher level (h-index) metrics as a proxy for the quality of research should understand that they are inherently biased due in part to:
Anyone using Journal Impact Factors as a proxy for quality or prestige of journals should understand that:
Moed, H. F. (2005). Citation analysis in research evaluation Dordrecht; Great Britain: Springer. DOI:10.1007/1-4020-3714-7
Wilsdon, J., et al. (2015). The Metric Tide: Report of the Independent Review of the Role of Metrics in Research Assessment and Management. Higher Education Funding Council for England (HEFCE). DOI: 10.13140/RG.2.1.4929.1363.