Trust Based Mechanism
Design
Use MD
Motivation
• Fuse the fields of trust-modelling and
mechanism design
• Trust measures how good an interaction
partner is
• Mechanism design concerns itself with the
allocation of resources.
Deciding on a service provider
Garage X is
better than Y
priceX
X
priceY
Garage Y is
better than X!
Y
Motivation
WHICH GARAGE TO
EMPLOY?
X
Y
Mechanism Design
• The design of systems of interacting
agents
– Protocols (allocation + payment schemes)
to ensure that certain global properties are
achieved at equilibrium (game-theoretic).
– Selfish, rational agents possessing private
information i.e. each agent maximises its
utility function given its type.
Task Allocation
• Agent 4 wants a task to be
completed.
• Problem
– Who to allocate task to ?
– How much to pay task performer ?
• Solution
- VCG mechanism
Applying VCG
(£210,task1)
Centre
Allocate task to 1,
4 pays 1 £50
4
Ask(£40,task1)
1
Ask(£80,task1)
2
Ask(£50,task1)
3
VCG Mechanism
• Class of mechanisms satisfying
– Efficiency – outcome maximising utility for all
agents
– Individually rationality – incentivise participation
– Incentive compatibility – incentivise truthful
report
under dominant strategy:
• Works by:
– Allocation : choose allocation maximising utility
– Payment : charge marginal utility contributed by
agent
Cheapest not always best …..
Adding uncertainties
• Suppose that agent 4 has formulated a
belief about the Probability of Success
(POS) of other agents.
Agent i
Ci
´ 4i
E[v4 (K ; µ4 )]
1
40
70.25
2
3
4
80
0.52
5
1.0
50
0.85
128.5
n/a
n/a
n/a
130
Trust
• So far : one’s own experience.
• Would be better to factor in others’
experiences as well.
– First time agent
– Greater pool of experience for seasoned user.
• Trust a way of aggregating everyone’s
experience
Our model of trust
• Each time a task is
performed, record POS
4
• Publicly transmit the POS
to other agents
2
• Trust
– Use a simple averaging
function over reports from
all agents
– Give more weight to more
trusted referrals, or agents
with similar properties
3
1
Record
Transmit
POS
POS
Incorporate Trust
Agent i
ci
1
´ 1i
´ 2i
´ 3i
t i4
40
0.4
1.0
0.85
0.5
65
2
80
0.6
1.0
0.9
1.0
130
3
50
0.5
1.0
0.9
0.87
132.7
4
1
0.525
1.0
0.85
na
na
® = [0:3 0:2 0:1 0:4]; v4 (¿) = 210
E[v4 (j ; µ4 )]
TBMD
(£210,task1)
Trust model
´ 4i
Centre
4
´ 3i
c1 = 40
´ 1i
1
c2 = 80 ´ i
2
c3 = 50
2
b¤
Kobserved
4 transmits
1,
Centre
2, 3 transmit
calculates
to centre
to optimal
centre
-- value
--allocation
cost
of task,
of doing
task and
POS
observed
and trust
POS
model
3
TBMD
(£210,task1)
Trust model
´ 4i
Centre
4
´ 3i
c1 = 40
´ 1i
1
c2 = 80 ´ i
2
c3 = 50
3
2
¤
Centre calculates optimal allocation K ¡ i without each agent i’s POS report.
Di = U( Kb ¤ ; :) ¡ U(K ¡¤ i ; :)
TBMD
(£210,task1)
Trust model
K ¤; ri
´ 4i
Centre
4
´ 3i
c1 = 40
´ 1i
1
c2 = 80 ´ i
2
2
Centre calculates transfer to i , r i = mci ¡ D i
c3 = 50
3
Steps in TBMD
Trust Based Mechanism Design
(TBMD)
• An individually rational and efficient
ICDR mechanism that can also
select those agents that are most
successful at their task
Experimental Setup
• Set of buyers and sellers.
• Biased (but truthful!) report from seller.
• Buyers attach different levels of
importance to seller’s report.
Results
Expected Value of Allocation
1
K*
0.95
K*TBM
0.9
K*TBM0.25
0.85
K*VCG
0.8
K*TBM0.5
0.75
1
101
201
301
Iteration
401
501
K*FTM
Conclusions
• Task allocation with uncertainty.
• Only incentive-compatible, individually-rational
efficient reputation mechanism.
• Works with a very large class of trust models.
End
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Any Questions?