  The most commonly used publication metrics for individual researchers are the
the total number of publications, the total number of citations, and Hirsch's
$h$-index. Each of these is cumulative, and hence increases throughout a
researcher's career, making it less suitable for evaluation of junior
researchers or assessing recent impact. Most other author-level measures in the
literature share this cumulative property. By contrast, we aim to study
non-cumulative measures that answer the question "in terms of citation impact,
what have you done lately?"
  We single out six measures from the rather sparse literature, including
Hirsch's $m$-index, a time-scaled version of the $h$-index. We introduce new
measures based on the idea of "citation acceleration". After presenting several
axioms for non-cumulative measures, we conclude that one of our new measures
has much better theoretical justification. We present a small-scale study of
its performance on real data and conclude that it shows substantial promise for
future use.
