The size of patent categories: USPTO 1976-2006

Categorization is an important phenomenon in science and society, and
classification systems reflect the mesoscale organization of knowledge.
The Yule-Simon-Naranan model, which assumes exponential growth of the
number of categories and exponential growth of individual categories
predicts a power law (Pareto) size distribution, and a power law
size-rank relation (Zipf’s law). However, the size distribution of
patent subclasses departs from a pure power law, and is shown to be
closer to a shifted power law. At a higher aggregation level (patent
classes), the rank-size relation deviates even more from a pure power
law, and is shown to be closer to a generalized beta curve. These
patterns can be explained by assuming a shifted exponential growth of
individual categories to obtain a shifted power law size distribution
(for subclasses), and by assuming an asymmetric logistic growth of the
number of categories to obtain a generalized beta size-rank relationship
(for classes). This may suggest a shift towards incremental more than
radical innovation.