Vocabulary growth in collaborative tagging systems

TL;DR

This study analyzes the growth of tags in del.icio.us, revealing power-law behaviors in vocabulary expansion that suggest underlying cognitive processes in collaborative tagging systems.

Contribution

It provides a large-scale empirical analysis of vocabulary growth, uncovering universal power-law behaviors in both global and local tag usage over time.

Findings

Vocabulary size grows as a power-law with time.

Growth exponents are less than one, indicating sub-linear growth.

Universal patterns observed across different resources and contexts.

Abstract

We analyze a large-scale snapshot of del.icio.us and investigate how the number of different tags in the system grows as a function of a suitably defined notion of time. We study the temporal evolution of the global vocabulary size, i.e. the number of distinct tags in the entire system, as well as the evolution of local vocabularies, that is the growth of the number of distinct tags used in the context of a given resource or user. In both cases, we find power-law behaviors with exponents smaller than one. Surprisingly, the observed growth behaviors are remarkably regular throughout the entire history of the system and across very different resources being bookmarked. Similar sub-linear laws of growth have been observed in written text, and this qualitative universality calls for an explanation and points in the direction of non-trivial cognitive processes in the complex interaction patterns characterizing collaborative tagging.