A two-dimensional index to quantify both scientific research impact and scope

Modern management of research is increasingly based on quantitative bibliometric indices. Nowadays, the h-index is a major measure of research output that has supplanted all other citation-based indices. In this context, indicators that complement the h-index by evaluating different facets of research achievement are compelling. As an additional bibliometric source that can be easily extracted from available databases, we propose to use the number of distinct journals Nj in which an individual's papers were published. We show that Nj is independent of citation counts, and argue that it is a relevant indicator of research scope, since it quantifies readership extent and scientific multidisciplinarity. Combining the h-index and Nj, we define a two-dimensional index (H,M) that measures both the output (through H) and the outreach (through M) of individual's research. In order to probe the relevance of this two dimensional index, we have analysed the scientific production of a panel of physicists belonging to the same Department but with different research themes. The analysis of bibliometric data confirms that the two indices are uncorrelated and shows that while H reliably ranks the impact of researchers, M accurately sorts multidisciplinary or readership aspects. We conclude that the two indices together offer a more complete picture of research performance and can be applied to either individuals, research groups or institutions.