Comparative Analysis of Patent Citations of Different Fields: In Consideration of the Data Size Dependency of Statistical Measures

This study focused on the classifications assigned to patents, and examined the number of different classifications of patents citing each patent, namely the diversity of citing fields. Specifically, we observed the expected values for growth in the average number of citing fields according to the increase in the data size (i.e., the cumulative number of citations) when regarding the observation period as being in a synchronic state and assuming that the strength of potential connections between each subject patent and citing fields is constant. A total of 347,327 patent applications published in Japan in 1993 were subjected to the analysis. The results of comparisons between fields showed that, irrespective of the cumulative number of citations, the transition of the diversity of citing fields is stable with higher values in the fields of “performing operations; transporting,” “chemistry; metallurgy,” and “textiles; paper” (sections B, C, and D of the International Patent Classification), while it is stable with lower values in the field of “fixed constructions” (section E). Moreover, as for the field of electricity (section H), it was found that the growth rate of the diversity of citing fields is markedly slow—in other words, a patent in this field tends to receive citations repeatedly from the same fields as before and to be hardly ever cited by patents belonging to fields different from them even when the cumulative number of citations increases.