K-Index as an Exploratory Quantitative Indicator of Competitive Structure: A Comparative Review of Japanese Industry Evidence

Porter’s Five Forces remains a foundational framework in strategic management, but its application is often qualitative and can be difficult to compare across industries shaped by digitalization, network effects, and intangible capital. This study examines the K-Index (K) through a comparative review of published estimates of scale elasticity across five Japanese industries: automotive, printing, retail, financial services, and information services. K is treated as a quantitative indicator of latent competitive structure, particularly related to entry barriers and concentration pressure. The reviewed evidence shows that information services exhibit the strongest positive K values, ranging from 1.197 to 1.643, and that the automotive industry also shows a positive K value of 0.695. Metropolitan retail tends toward positive K, whereas printing, rural retail, and financial services remain in the non-positive-K category. These patterns suggest that K may complement Porter’s framework as an indicator for comparing competitive structures across industries. To be cautious, this review does not constitute an external empirical validation of K. The findings, including those about positive- and non-positive-K categories, should be interpreted as exploratory because the available evidence is limited to a small number of Japanese sectors. Further research should expand the sectoral and international scope and examine how changes in K are associated with observed competitive outcomes, such as concentration, pricing, entry, and innovation.