x- and z-indices, and number of citations per paper in a selection of countries.

The 20 countries with the highest number of citations in the Essential Science Indicators of the ISI Web of Knowledge. The x-index is calculated from the mean of the yearly values of N1 and N0.1 from 2003 to 2007, and the z-index is the x-index divided by the mean of the number of national articles in these years. The number of citations per paper is taken from the Essential Science Indicators in All Fields.

本数据集涵盖在ISI知识网（ISI Web of Knowledge）的基本科学指标（Essential Science Indicators）中被引频次最高的20个国家。x指数由2003年至2007年间N1与N0.1的年度数值的平均值计算得到，z指数则为x指数除以同期各国论文总数量的平均值。单篇论文被引频次数据取自全学科领域的基本科学指标（Essential Science Indicators）。