The changes of average impact factor per paper and their relationship to number of publications. Publication data were collected as described in materials and methods section. The impact factor per paper was calculated by dividing total impact factor by total number of published papers in each year. Linear regression was used to analyze the significant changes of impact factor per paper over the study period (Figure 2a). For correlation of impact factor per paper with the number of publication, Two-tailed Pearson test was used to determine significance (Figure 2b). Part of data (2003–2011) for mainland China in 3a was reported earlier .