  As a simple means for comparing and - if possible - predicting scientific
impacts of different researchers working in the same field, we suggest
comparing their "sales curves". A sales curve is the number of citations of the
researcher's papers per year, the citation rate, considered as a function of
time. As examples, we present the citation histories of 10 older well-cited
scientists working in the same field. The sales curve is found to be highly
individual, that is, there is a large variation between different scientists'
sales curves. For each well-cited scientist, however, the sales curve is
steadily increasing as long as he is young and active, and its slope, the
citation acceleration, contains the essential information about his impact. The
slope averaged over the time of activity of the scientist is roughly
independent of time and is a fairly age-independent measure of his scientific
impact. In physics and chemistry, well-cited active scientists have
time-averaged citation accelerations at the order of 10 citations per year^2 or
more. The normal citation acceleration is an order of magnitude smaller. We
also show the sales curves for three large research institutes whose sizes have
been fairly constant over the last 35 years. These sales curves are quite
linear and have slopes at the order of 1 citation per scientist per year^2.
