While concentration ratios within particular industries are accepted as conventional indicators of market power, the use of an "overall concentration" approach has been viewed with suspicion. This approach defines overall concentration as being the proportion of some aggregate of economic activity accounted for by a relatively small number of the largest firms taken together.
Jacquemin, A. P., & Kumps, A.-M. (1971). Changes in the Size Structure of the Largest European Firms : An Entropy Measure (Working Paper Institut des sciences écnomiques 7105). https://hdl.handle.net/2078.5/275491