<s>
XML	B-Language
retrieval	I-Language
,	O
or	O
XML	B-Protocol
information	B-Library
retrieval	I-Library
,	O
is	O
the	O
content-based	O
retrieval	O
of	O
documents	O
structured	O
with	O
XML	B-Protocol
(	O
eXtensible	B-Protocol
Markup	I-Protocol
Language	I-Protocol
)	O
.	O
</s>
<s>
As	O
such	O
it	O
is	O
used	O
for	O
computing	O
relevance	O
of	O
XML	B-Protocol
documents	I-Protocol
.	O
</s>
<s>
Most	O
XML	B-Language
retrieval	I-Language
approaches	O
do	O
so	O
based	O
on	O
techniques	O
from	O
the	O
information	B-Library
retrieval	I-Library
(	O
IR	O
)	O
area	O
,	O
e.g.	O
</s>
<s>
by	O
computing	O
the	O
similarity	O
between	O
a	O
query	B-Library
consisting	O
of	O
keywords	O
(	O
query	B-Library
terms	O
)	O
and	O
the	O
document	O
.	O
</s>
<s>
However	O
,	O
in	O
XML-Retrieval	B-Language
the	O
query	B-Library
can	O
also	O
contain	O
structural	B-General_Concept
hints	B-Language
.	O
</s>
<s>
So-called	O
"	O
content	O
and	O
structure	O
"	O
(	O
CAS	O
)	O
queries	B-Library
enable	O
users	O
to	O
specify	O
what	O
structure	O
the	O
requested	O
content	O
can	O
or	O
must	O
have	O
.	O
</s>
<s>
Taking	O
advantage	O
of	O
the	O
self-describing	O
structure	O
of	O
XML	B-Protocol
documents	I-Protocol
can	O
improve	O
the	O
search	O
for	O
XML	B-Protocol
documents	I-Protocol
significantly	O
.	O
</s>
<s>
This	O
includes	O
the	O
use	O
of	O
CAS	O
queries	B-Library
,	O
the	O
weighting	O
of	O
different	O
XML	B-Protocol
elements	O
differently	O
and	O
the	O
focused	O
retrieval	O
of	O
subdocuments	O
.	O
</s>
<s>
Ranking	O
in	O
XML-Retrieval	B-Language
can	O
incorporate	O
both	O
content	O
relevance	O
and	O
structural	B-General_Concept
similarity	O
,	O
which	O
is	O
the	O
resemblance	O
between	O
the	O
structure	O
given	O
in	O
the	O
query	B-Library
and	O
the	O
structure	O
of	O
the	O
document	O
.	O
</s>
<s>
Also	O
,	O
the	O
retrieval	O
units	O
resulting	O
from	O
an	O
XML	B-Protocol
query	B-Library
may	O
not	O
always	O
be	O
entire	O
documents	O
,	O
but	O
can	O
be	O
any	O
deeply	O
nested	O
XML	B-Protocol
elements	O
,	O
i.e.	O
</s>
<s>
The	O
INitiative	O
for	O
the	O
Evaluation	O
of	O
XML-Retrieval	B-Language
(	O
INEX	O
)	O
was	O
founded	O
in	O
2002	O
and	O
provides	O
a	O
platform	O
for	O
evaluating	O
such	O
algorithms	O
.	O
</s>
<s>
Three	O
different	O
areas	O
influence	O
XML-Retrieval	B-Language
:	O
</s>
<s>
Query	B-Language
languages	I-Language
such	O
as	O
the	O
W3C	O
standard	O
XQuery	B-Language
supply	O
complex	O
queries	B-Library
,	O
but	O
only	O
look	O
for	O
exact	O
matches	O
.	O
</s>
<s>
Most	O
XML-centered	O
approaches	O
imply	O
a	O
quite	O
exact	O
knowledge	O
of	O
the	O
documents	O
 '	O
schemas	B-Application
.	O
</s>
<s>
Classic	O
database	O
systems	O
have	O
adopted	O
the	O
possibility	O
to	O
store	O
semi-structured	B-General_Concept
data	I-General_Concept
and	O
resulted	O
in	O
the	O
development	O
of	O
XML	B-Application
databases	I-Application
.	O
</s>
<s>
Often	O
,	O
they	O
are	O
very	O
formal	O
,	O
concentrate	O
more	O
on	O
searching	O
than	O
on	O
ranking	O
,	O
and	O
are	O
used	O
by	O
experienced	O
users	O
able	O
to	O
formulate	O
complex	O
queries	B-Library
.	O
</s>
<s>
Classic	O
information	B-Library
retrieval	I-Library
models	O
such	O
as	O
the	O
vector	O
space	O
model	O
provide	O
relevance	O
ranking	O
,	O
but	O
do	O
not	O
include	O
document	O
structure	O
;	O
only	O
flat	O
queries	B-Library
are	O
supported	O
.	O
</s>
<s>
They	O
can	O
be	O
extended	O
to	O
consider	O
structural	B-General_Concept
information	O
and	O
dynamic	O
document	B-Algorithm
retrieval	I-Algorithm
.	O
</s>
<s>
Examples	O
for	O
approaches	O
extending	O
the	O
vector	O
space	O
models	O
are	O
available	O
:	O
they	O
use	O
document	O
subtrees	B-Application
(	O
index	O
terms	O
plus	O
structure	O
)	O
as	O
dimensions	O
of	O
the	O
vector	O
space	O
.	O
</s>
<s>
For	O
data-centric	O
XML	B-Protocol
datasets	O
,	O
the	O
unique	O
and	O
distinct	O
keyword	O
search	O
method	O
,	O
namely	O
,	O
XDMA	O
for	O
XML	B-Application
databases	I-Application
is	O
designed	O
and	O
developed	O
based	O
on	O
dual	O
indexing	O
and	O
mutual	O
summation	O
.	O
</s>
