Trust and Technology - Trust Theory: A Socio-Cognitive and Computational Model - page 341

Dempster and Shafer (Shafer, 1976) to cope with these limits are presented in (Motro and
Smets, 1997). However, given his direct work on trust and reputation models, it is of interest
to cite Josang's approach (Josang, 2001) that introduces the subjective logic : an attempt to
overcome the limits of the classical logics and also taking into consideration the uncertainty,
ignorance and the subjective characteristic of the beliefs.
This approach is strongly influenced by Dempster and Shafer's work but with some specific
interesting intuitions. The opinions in Josang's approach are belief/trust metrics denoted by:

A
x

ω = ( b , d , u , a ) (12.1)

x expresses the trust of agent A about the truth of the statement of x . b represents the
positive A 's beliefs, d represents the A 's disbeliefs, u represents the uncertainty with:

where ω

b , d , u ∈ [0 , 1] b + d + u = 1 a ∈ [0 , 1]

The parameter a is called relative atomicity and represents the base rate probability in the
absence of evidence: it determines how uncertainty contributes to an opinion's probability
expectation value E (

x ):

ω

E ω

x =

A

+

b au (12.2)

The subjective logics introduce two operators ( discounting and consensus ) useful for trust
derivation from other opinions.

12.2.2 Computational Approach
The computational approach to trust has as a main goal the implementation of a trust model
in an automatic system independent from the representational framework. The computational
trust models can follow different principles on the basis of the adopted approaches with
respect to:

the sources on which the model evaluates the trustee's trustworthiness,
the kind of metric for trust measures.

Different Kinds of Sources

With respect to the trust sources, as shown in Chapter 6, we can distinguish among direct
experience and indirect experience . Direct experience is the more simple and elementary
agent's source deriving from its previous direct experiences with other agents and with the
world; it strongly depends on the agent's perceptive apparatus: the kind of input it is able to
perceive.
Indirect experience sources can be, in their turn, articulated in the so-called reputation
(others' experience directly communicated or made available (and possibly mediated) by
some central or decentral mechanism) and types of general reasoning and deduction , like
inference over categories, classes of agents, situations (scripts), and so on.
The sources based on the reasoning and their influences on the trust decision are par-
tially shown in Chapter 6 of this topic. Reputation has become a very diffused and practiced