MERCREDI 17 JUIN 2009
14 heures (Attention: horaire inhabituel!)
Séminaire d'Intérêt Général
Salle Séminaire 5
Centre de Physique Théorique
Marseille-Luminy

Vittorio Loreto
Dipartimento di Fisica, "Sapienza" Universita' di
Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy
and
Complex Networks Lagrange Laboratory,
Institute for Scientific Interchange (ISI), Torino, Italy

Title: Collective dynamics of social annotation

Abstract: The enormous increase of popularity and use of the WWW has
led in the recent years to important changes in the ways people
communicate. An interesting example of this fact is provided by the
now very popular social annotation systems, through which users
annotate resources (such as web pages or digital photographs) with
text keywords dubbed tags. Collaborative tagging has been quickly
gaining ground because of its ability to recruit the activity of web
users into effectively organizing and sharing vast amounts of
information. Understanding the rich emerging structures resulting from
the uncoordinated actions of users calls for an interdisciplinary
effort. In particular concepts borrowed from statistical physics, such
as random walks, and the complex networks framework, can effectively
contribute to the mathematical modeling of social annotation systems.
First I'll introduce a stochastic model of user behavior embodying two
main aspects of collaborative tagging: (i) a frequency-bias mechanism
related to the idea that users are exposed to each other's tagging
activity; (ii) a notion of memory, or aging of resources, in the form
of a heavy-tailed access to the past state of the system. Remarkably,
this simple modeling is able to account quantitatively for the
observed experimental features with a surprisingly high accuracy. This
points in the direction of a universal behavior of users who, despite
the complexity of their own cognitive processes and the uncoordinated
and selfish nature of their tagging activity, appear to follow simple
activity patterns. Next I'll show how the process of social annotation
can be seen as a collective but uncoordinated exploration of an
underlying semantic space, pictured as a graph, through a series of
random walks. This modeling framework reproduces several aspects, so
far unexplained, of social annotation, among which the peculiar growth
of the size of the vocabulary used by the community and its complex
network structure that represents an externalization of semantic
structures grounded in cognition and typically hard to access.