UC Santa Cruz ranks 21st in the world in a five-year measure of the top 500 universities based on the impact of its researchers' scholarly publications.

The Leiden Rankings for 2011-2012 by the Centre for Science and Technologies Studies at Leiden University in the Netherlands is based on an analysis of publications in Thomson Reuters' Web of Science database from 2005-2009. The study measures the scientific impact of university research and of its contribution to scientific collaboration. Only publications in the sciences and social sciences are included.

The ranking is based on how many times a particular publication – journal article, letter, or review – is cited by other researchers. It is an effort to provide highly accurate measurements of the scientific impact of universities. Rankings are based on more than 25 years of analyzing publications by Leiden University.

The study looked at universities worldwide that had at least 500 publications during the period in an effort to take into account the size of a university. Sheer numbers of publications doesn't count as much as the collective impact of individual publications. Particularly important was the proportion of a university's publications that, when compared with other similar publications, belong in the top 10 percent of the most frequently cited.

For instance, it looked at the impact of 2,217 publications in which UCSC researchers participated during that period and said 17.4 percent of those ranked in the top 10 of the most frequently cited. Over the same time period, UCLA researchers had nearly eight times the publications and ranked 17.5 percent of the top 10.

Ranked highest in the world was MIT. Other UCs in the rankings are Santa Barbara (7), Berkeley (8), San Francisco (10), San Diego (19),  Los Angeles (20), Riverside (29), and Irvine (37).