The research interests and preferences of the reader communities associated to any given digital library may change over the course of years. It is vital for digital library services and collection management to be informed of such changes, and to determine how they may point to future trends. We propose the Impact Discrepancy Ratio metric for the detection of research trends in a large digital library by comparing a reader-defined metric of journal impact to the Institute for Scientific Information Impact Factor (ISI IF) over the course of three years. An analysis for the Los Alamos National Laboratory (LANL) Research Library (RL) comparing reader impact to the ISI IF for 1998 and 2001 indicates journals relating to climatology have undergone a sharp increase in local impact. This evolution pinpoints specific shifts in the local strategies and reader interests of the LANL RL which were qualitatively validated by LANL RL management.
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