  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.
