  A researcher collaborating with many groups will normally have more papers
(and thus higher citations and $h$-index) than a researcher spending all
his/her time working alone or in a small group. While analyzing an author's
research merit, it is therefore not enough to consider only the collective
impact of the published papers, it is also necessary to quantify his/her share
in the impact. For this quantification, here I propose the $I$-index which is
defined as an author's percentage share in the total citations that his/her
papers have attracted. It is argued that this $I$-index does not directly
depend on the most of the subjective issues like an author's influence,
affiliation, seniority or career break. A simple application of the Central
Limit Theorem shows that, the scheme of equidistribution of credit among the
coauthors of a paper will give us the most probable value of the $I$-index
(with an associated small standard deviation which decreases with increasing
$h$-index). I show that the total citations ($N_c$), the $h$-index and the
$I$-index are three independent parameters (within their bounds), and together
they give a comprehensive idea of an author's overall research performance.
