Page 298 - Becoming Metric Wise

290 Becoming Metric-Wise

discovery or research for which they won the Nobel Prize—occurred
early in their life. In a study by Kanazawa (2003) the author found
that nearly a quarter of all scientists makes their most significant contri-
bution of their career during the 5 years around age 30. A similar obser-
vation had been made 20 years earlier by Zhao and Jiang (1986). In
collaboration with Liang et al. (1996) Zhao found that these peak
achievements (1928 major scientific and technological inventions
from the year 1500 to the year 1960) follow a Weibull distribution. In
this set of major achievements the peak occurred between the ages of
31 and 40.
There is, however, a difference between good, even eminent work
and peak performances. Dennis (1956), who studied 156 19th century
scientists who lived at least till the age of 70, already found that, on
the whole, there was little change in mean output of scientific articles
between the ages of 30 and 60. Cole (1979) in a study of the number
of published articles in a 25-year period and the number of received
citations found that researchers who received professional recognition,
namely citations, for their earlier work had the greatest inclination to
be productive at a later stage. In this way productivity at later age was
linked to a form of cumulative advantage. Kyvik (1990) studied more
recent data and found a productivity peak among Norwegian scien-
tists in the 45 49 age group (applying a kind of fractionalization and
considering books as an equivalent of 2 to 6 articles, depending on
the type and number of pages). This was, however, just an average
over all fields. He found that e.g., in the social sciences productivity
remained more or less at the same level for all age groups, while
in the natural sciences productivity continually decreased with
increasing age.
Costas et al. (2010) divided scientists working at the Spanish National
Research Council (CSIC) into three professional categories: tenured
scientists, research scientists and research professors. In each professional
category and for each field studied they found that the younger scientists
performed best as measured by productivity, received citations and
impact-based indicators.
When studying the possible decline in intellectual powers or original-
ity over the years, biological age is used. Yet, when the relation between
age and one’s career is the topic of study, it is better to consider career
age, usually counted from the year a Ph.D is obtained.