In this study the amount of “informal” citations (i.e. those mentioning only author names or their initials instead of the
complete references) in comparison to the “formal” (full reference based) citations is analyzed using some pioneers of chemistry
and physics as examples. The data reveal that the formal citations often measure only a small fraction of the overall impact
of seminal publications. Furthermore, informal citations are mainly given instead of (and not in addition to) formal citations.
As a major consequence, the overall impact of pioneering articles and researchers cannot be entirely determined by merely
counting the full reference based citations.