  Citation measures, and newer altmetric measures such as downloads are now
commonly used to inform personnel decisions. How well do or can these measures
measure or predict the past, current of future scholarly performance of an
individual? Using data from the Smithsonian/NASA Astrophysics Data System we
analyze the publication, citation, download, and distinction histories of a
cohort of 922 individuals who received a U.S. PhD in astronomy in the period
1972-1976. By examining the same and different measures at the same and
different times for the same individuals we are able to show the capabilities
and limitations of each measure. Because the distributions are lognormal
measurement uncertainties are multiplicative; we show that in order to state
with 95% confidence that one person's citations and/or downloads are
significantly higher than another person's, the log difference in the ratio of
counts must be at least 0.3 dex, which corresponds to a multiplicative factor
of two.
