<s>
Truth	B-General_Concept
discovery	I-General_Concept
(	O
also	O
known	O
as	O
truth	O
finding	O
)	O
is	O
the	O
process	O
of	O
choosing	O
the	O
actual	O
true	O
value	O
for	O
a	O
data	B-General_Concept
item	I-General_Concept
when	O
different	O
data	O
sources	O
provide	O
conflicting	O
information	O
on	O
it	O
.	O
</s>
<s>
Truth	B-General_Concept
discovery	I-General_Concept
problems	O
can	O
be	O
divided	O
into	O
two	O
sub-classes	O
:	O
single-truth	O
and	O
multi-truth	O
.	O
</s>
<s>
In	O
the	O
first	O
case	O
only	O
one	O
true	O
value	O
is	O
allowed	O
for	O
a	O
data	B-General_Concept
item	I-General_Concept
(	O
e.g	O
birthday	O
of	O
a	O
person	O
,	O
capital	O
city	O
of	O
a	O
country	O
)	O
.	O
</s>
<s>
Typically	O
,	O
truth	B-General_Concept
discovery	I-General_Concept
is	O
the	O
last	O
step	O
of	O
a	O
data	B-General_Concept
integration	I-General_Concept
pipeline	O
,	O
when	O
the	O
schemas	B-Application
of	O
different	O
data	O
sources	O
have	O
been	O
unified	B-General_Concept
and	O
the	O
records	O
referring	O
to	O
the	O
same	O
data	B-General_Concept
item	I-General_Concept
have	O
been	O
detected	O
.	O
</s>
<s>
The	O
abundance	O
of	O
data	O
available	O
on	O
the	O
web	O
makes	O
more	O
and	O
more	O
probable	O
to	O
find	O
that	O
different	O
sources	O
provide	O
(	O
partially	O
or	O
completely	O
)	O
different	O
values	O
for	O
the	O
same	O
data	B-General_Concept
item	I-General_Concept
.	O
</s>
<s>
This	O
,	O
together	O
with	O
the	O
fact	O
that	O
we	O
are	O
increasing	O
our	O
reliance	O
on	O
data	O
to	O
derive	O
important	O
decisions	O
,	O
motivates	O
the	O
need	O
of	O
developing	O
good	O
truth	B-General_Concept
discovery	I-General_Concept
algorithms	O
.	O
</s>
<s>
Many	O
currently	O
available	O
methods	O
rely	O
on	O
a	O
voting	O
strategy	O
to	O
define	O
the	O
true	O
value	O
of	O
a	O
data	B-General_Concept
item	I-General_Concept
.	O
</s>
<s>
Nevertheless	O
,	O
recent	O
studies	O
,	O
have	O
shown	O
that	O
,	O
if	O
we	O
rely	O
only	O
on	O
majority	O
voting	O
,	O
we	O
could	O
get	O
wrong	O
results	O
even	O
in	O
30%	O
of	O
the	O
data	B-General_Concept
items	I-General_Concept
.	O
</s>
<s>
Ideally	O
,	O
supervised	B-General_Concept
learning	I-General_Concept
techniques	O
could	O
be	O
exploited	O
to	O
assign	O
a	O
reliability	O
score	O
to	O
sources	O
after	O
hand-crafted	O
labeling	O
of	O
the	O
provided	O
values	O
;	O
unfortunately	O
,	O
this	O
is	O
not	O
feasible	O
since	O
the	O
number	O
of	O
needed	O
labeled	O
examples	O
should	O
be	O
proportional	O
to	O
the	O
number	O
of	O
sources	O
,	O
and	O
in	O
many	O
applications	O
the	O
number	O
of	O
sources	O
can	O
be	O
prohibitive	O
.	O
</s>
<s>
only	O
one	O
true	O
value	O
is	O
allowed	O
for	O
each	O
data	B-General_Concept
item	I-General_Concept
;	O
</s>
<s>
different	O
values	O
provided	O
for	O
a	O
given	O
data	B-General_Concept
item	I-General_Concept
oppose	O
to	O
each	O
other	O
;	O
</s>
<s>
claiming	O
one	O
value	O
for	O
a	O
given	O
data	B-General_Concept
item	I-General_Concept
does	O
not	O
imply	O
opposing	O
to	O
all	O
the	O
other	O
values	O
;	O
</s>
<s>
the	O
number	O
of	O
true	O
values	O
for	O
each	O
data	B-General_Concept
item	I-General_Concept
is	O
not	O
known	O
a	O
priori	O
.	O
</s>
<s>
The	O
vast	O
majority	O
of	O
truth	B-General_Concept
discovery	I-General_Concept
methods	O
are	O
based	O
on	O
a	O
voting	O
approach	O
:	O
each	O
source	O
votes	O
for	O
a	O
value	O
of	O
a	O
certain	O
data	B-General_Concept
item	I-General_Concept
and	O
,	O
at	O
the	O
end	O
,	O
the	O
value	O
with	O
the	O
highest	O
vote	O
is	O
select	O
as	O
the	O
true	O
one	O
.	O
</s>
<s>
At	O
each	O
step	O
of	O
the	O
truth	B-General_Concept
discovery	I-General_Concept
algorithm	O
the	O
trustworthiness	O
score	O
of	O
each	O
data	O
source	O
is	O
refined	O
,	O
improving	O
the	O
assessment	O
of	O
the	O
true	O
values	O
that	O
in	O
turn	O
leads	O
to	O
a	O
better	O
estimation	O
of	O
the	O
trustworthiness	O
of	O
the	O
sources	O
.	O
</s>
<s>
Detecting	O
copying	O
behaviors	O
is	O
very	O
important	O
,	O
in	O
fact	O
,	O
copy	O
allows	O
to	O
spread	O
false	O
values	O
easily	O
making	O
truth	B-General_Concept
discovery	I-General_Concept
very	O
hard	O
,	O
since	O
many	O
sources	O
would	O
vote	O
for	O
the	O
wrong	O
values	O
.	O
</s>
<s>
Most	O
of	O
the	O
currently	O
available	O
truth	B-General_Concept
discovery	I-General_Concept
methods	O
have	O
been	O
designed	O
to	O
work	O
well	O
only	O
in	O
the	O
single-truth	O
case	O
.	O
</s>
<s>
These	O
methods	O
estimate	O
source	O
trustworthiness	O
exploiting	O
a	O
similar	O
technique	O
to	O
the	O
one	O
used	O
to	O
measure	O
authority	B-Application
of	O
web	O
pages	O
based	O
on	O
web	O
links	O
.	O
</s>
<s>
These	O
methods	O
estimate	O
source	O
trustworthiness	O
using	O
similarity	O
measures	O
typically	O
used	O
in	O
information	B-Library
retrieval	I-Library
.	O
</s>
<s>
where	O
is	O
a	O
value	O
provided	O
for	O
a	O
data	B-General_Concept
item	I-General_Concept
and	O
is	O
the	O
set	O
of	O
the	O
observed	O
values	O
provided	O
by	O
all	O
the	O
sources	O
for	O
that	O
specific	O
data	B-General_Concept
item	I-General_Concept
.	O
</s>
<s>
In	O
this	O
case	O
,	O
since	O
there	O
could	O
be	O
multiple	O
true	O
values	O
for	O
each	O
data	B-General_Concept
item	I-General_Concept
,	O
and	O
sources	O
can	O
provide	O
multiple	O
values	O
for	O
a	O
single	O
data	B-General_Concept
item	I-General_Concept
,	O
it	O
is	O
not	O
possible	O
to	O
consider	O
values	O
individually	O
.	O
</s>
<s>
These	O
methods	O
use	O
probabilistic	O
graphical	O
models	O
to	O
automatically	O
define	O
the	O
set	O
of	O
true	O
values	O
of	O
given	O
data	B-General_Concept
item	I-General_Concept
and	O
also	O
to	O
assess	O
source	O
quality	O
without	O
need	O
of	O
any	O
supervision	O
.	O
</s>
<s>
Many	O
real-world	O
applications	O
can	O
benefit	O
from	O
the	O
use	O
of	O
truth	B-General_Concept
discovery	I-General_Concept
algorithms	O
.	O
</s>
<s>
Typical	O
domains	O
of	O
application	O
include	O
:	O
healthcare	O
,	O
crowd/social	O
sensing	O
,	O
crowdsourcing	O
aggregation	O
,	O
information	B-General_Concept
extraction	I-General_Concept
and	O
knowledge	O
base	O
construction	O
.	O
</s>
<s>
Truth	B-General_Concept
discovery	I-General_Concept
algorithms	O
could	O
be	O
also	O
used	O
to	O
revolutionize	O
the	O
way	O
in	O
which	O
web	O
pages	O
are	O
ranked	O
in	O
search	B-Application
engines	I-Application
,	O
going	O
from	O
current	O
methods	O
based	O
on	O
link	B-Application
analysis	I-Application
like	O
PageRank	B-Algorithm
,	O
to	O
procedures	O
that	O
rank	O
web	O
pages	O
based	O
on	O
the	O
accuracy	O
of	O
the	O
information	O
they	O
provide	O
.	O
</s>
