Restricting the h-index to a publication and citation time window: A case study of a timed Hirsch index

The h-index has been shown to increase in many cases mostly because of citations to rather old publications. This inertia can be circumvented by restricting the evaluation to a citation time window. Here I report results of an empirical study analyzing the evolution of the thus defined timed h-index in dependence on the length of the citation time window. 1. Introduction A recently popular measure of scientific achievements of a researcher is given by the Hirsch index or h-index as defined by Hirsch (2005). It can easily be determined after sorting the publications of a researcher in decreasing order according to their citation frequencies. The index is then given by the largest rank h which is equal to or smaller than the number of citations to the h-th paper. It is nowadays often used as a performance indicator for evaluation purposes and fund allocations. Beside principle questions about the usefulness of single number indicators for such purposes there occurs a specific problem when the h-index is applied in such away. This is due to the fact that the index cannot decrease and that its increase is often dominated by citations to rather old publications. This inert behavior of the index (Schreiber, 2013) has raised doubts whether the h-index is a reasonable measure for predicting future scientific impact. Specifically, observed high correlations between h-index values of researchers in early and late stages of their careers (Hirsch, 2007) can be explained as structural correlations due to an order restriction (García-Pérez & Núñez-Antón, 2013), because the h-index cannot decrease. A solution could be the restriction of the determination of the h-index to recent publications in order to measure the recent performance of a researcher. I have recently analyzed citation records in such away, namely by discarding older publications and taking only papers published in the last years into account (Schreiber, 2014). In the present investigation I analyze my own citation data in a retrospective way, namely determining the evolution of the thus restrictively defined timed h-index h t in dependence on the length t of the utilized citation time window. These proceedings allow me to obtain insights into the changing impact of my publications during my career. For the same purpose, Pan & Fortunato (2014) have proposed the Author Impact Factor which extends the Journal Impact Factor to individual authors. Using a 5-year publication window, they described trends and variations of the impact …