Measure the impact of scientific research of a country is of great interest to policy makers in order to define both the amount of its spending and its priorities. To study in a systematic way the scientific impact of nations, using simple and relevant indicators,  data on the number of articles and citations produced by individual countries have been used in the past. Articles and citations are, in fact, an indirect measure of the output of the investment in research: the number of scientific articles is related to the activity carried out and the number of citations received by these articles measures popularity that one can consider correlated to the scientific quality. While, when referring to a single researcher these numbers should be treated with great caution, when considering the production of a whole country one can reasonably assume, thanks to the large numbers involved, that there is a proportionality between the total number of articles and citations and the global significance of the research.

Robert May was among the first to perform this type of analysis for the years 1981-1994; in particular compared the investments and the results of scientific research in various countries. Later, David King presented in 2004 a similar  but more refined analysis, for the years 1993-2002. More recently, other studies, several national and international agencies, using a similar methodology, measured the productivity of scientific research of nations by normalizing the number of scientific articles and citations received to the spending on research and university. In fact, when comparing very different countries (for example, the United States and Switzerland) it is necessary to take into account the fact that the global scientific production depends on the size of the country itself: for example, the number of researchers or total investment in research. As the number of researchers is not simply measurable (for example, in many countries there is a nontrivial problem in the census of not permanent researchers), it can be used as an indicator for comparing different countries, the spending on scientific research and development or the cost of higher education for research and development (the so-called HERD) that is surveyed by the OECD.

The political implications of this analysis are of two types. On the one hand we see the nations that are technology leaders, have the largest production of scientific articles and the highest number of citations. These countries also have the highest fraction of spending on research and development relative to GDP (almost 3%). On the other side, a comparative analysis of the impact of the different research areas can provide information about what is the most efficient way to implement your own search system: to specialize in certain areas of science or rather diversify as much as possible? In many countries is taking place in recent years, a worrying tendency to concentrate resources on a few centers of excellence and scientific projects, both decided in a top-down manner. To assess whether the latter is a policy that increases the efficiency of the system can be observed what happened in different countries over the past two decades, since when databases of publications and citations are available.

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