<s>
Knowledge-based	B-General_Concept
recommender	I-General_Concept
systems	I-General_Concept
(	O
knowledge	B-General_Concept
based	I-General_Concept
recommenders	I-General_Concept
)	O
are	O
a	O
specific	O
type	O
of	O
recommender	B-Application
system	I-Application
that	O
are	O
based	O
on	O
explicit	O
knowledge	O
about	O
the	O
item	O
assortment	O
,	O
user	O
preferences	O
,	O
and	O
recommendation	O
criteria	O
(	O
i.e.	O
,	O
which	O
item	O
should	O
be	O
recommended	O
in	O
which	O
context	O
)	O
.	O
</s>
<s>
These	O
systems	O
are	O
applied	O
in	O
scenarios	O
where	O
alternative	O
approaches	O
such	O
as	O
collaborative	B-Algorithm
filtering	I-Algorithm
and	O
content-based	O
filtering	O
cannot	O
be	O
applied	O
.	O
</s>
<s>
A	O
major	O
strength	O
of	O
knowledge-based	B-General_Concept
recommender	I-General_Concept
systems	I-General_Concept
is	O
the	O
non-existence	O
of	O
cold	B-General_Concept
start	I-General_Concept
(	O
ramp-up	O
)	O
problems	O
.	O
</s>
<s>
Knowledge-based	B-General_Concept
recommender	I-General_Concept
systems	I-General_Concept
are	O
well	O
suited	O
to	O
complex	O
domains	O
where	O
items	O
are	O
not	O
purchased	O
very	O
often	O
,	O
such	O
as	O
apartments	O
and	O
cars	O
.	O
</s>
<s>
Further	O
examples	O
of	O
item	O
domains	O
relevant	O
for	O
knowledge-based	B-General_Concept
recommender	I-General_Concept
systems	I-General_Concept
are	O
financial	O
services	O
,	O
digital	O
cameras	O
,	O
and	O
tourist	O
destinations	O
.	O
</s>
<s>
Rating-based	B-Application
systems	I-Application
often	O
do	O
not	O
perform	O
well	O
in	O
these	O
domains	O
due	O
to	O
the	O
low	O
number	O
of	O
available	O
ratings	O
.	O
</s>
<s>
In	O
this	O
context	O
,	O
the	O
recommender	B-Application
system	I-Application
must	O
take	O
into	O
account	O
constraints	O
:	O
for	O
instance	O
,	O
only	O
those	O
financial	O
services	O
that	O
support	O
the	O
investment	O
period	O
specified	O
by	O
the	O
customer	O
should	O
be	O
recommended	O
.	O
</s>
<s>
Neither	O
of	O
these	O
aspects	O
are	O
supported	O
by	O
approaches	O
such	O
as	O
collaborative	B-Algorithm
filtering	I-Algorithm
and	O
content-based	O
filtering	O
.	O
</s>
<s>
Knowledge-based	B-General_Concept
recommender	I-General_Concept
systems	I-General_Concept
are	O
often	O
conversational	O
,	O
i.e.	O
,	O
user	O
requirements	O
and	O
preferences	O
are	O
elicited	O
within	O
the	O
scope	O
of	O
a	O
feedback	O
loop	O
.	O
</s>
<s>
A	O
major	O
reason	O
for	O
the	O
conversational	O
nature	O
of	O
knowledge-based	B-General_Concept
recommender	I-General_Concept
systems	I-General_Concept
is	O
the	O
complexity	O
of	O
the	O
item	O
domain	O
where	O
it	O
is	O
often	O
impossible	O
to	O
articulate	O
all	O
user	O
preferences	O
at	O
once	O
.	O
</s>
<s>
In	O
a	O
search-based	O
recommender	B-Application
,	O
user	O
feedback	O
is	O
given	O
in	O
terms	O
of	O
answers	O
to	O
questions	O
which	O
restrict	O
the	O
set	O
of	O
relevant	O
items	O
.	O
</s>
<s>
On	O
the	O
technical	O
level	O
,	O
search-based	O
recommendation	O
scenarios	O
can	O
be	O
implemented	O
on	O
the	O
basis	O
of	O
constraint-based	O
recommender	B-Application
systems	I-Application
.	O
</s>
<s>
Constraint-based	O
recommender	B-Application
systems	I-Application
are	O
implemented	O
on	O
the	O
basis	O
of	O
constraint	O
search	O
or	O
different	O
types	O
of	O
conjunctive	O
query-based	O
approaches	O
.	O
</s>
<s>
In	O
a	O
navigation-based	O
recommender	B-Application
,	O
user	O
feedback	O
is	O
typically	O
provided	O
in	O
terms	O
of	O
"	O
critiques	O
"	O
which	O
specify	O
change	O
requests	O
regarding	O
the	O
item	O
currently	O
recommended	O
to	O
the	O
user	O
.	O
</s>
