<s>
Document	B-Algorithm
retrieval	I-Algorithm
is	O
defined	O
as	O
the	O
matching	O
of	O
some	O
stated	O
user	O
query	B-Library
against	O
a	O
set	O
of	O
free-text	B-Application
records	O
.	O
</s>
<s>
User	O
queries	B-Library
can	O
range	O
from	O
multi-sentence	O
full	O
descriptions	O
of	O
an	O
information	O
need	O
to	O
a	O
few	O
words	O
.	O
</s>
<s>
Document	B-Algorithm
retrieval	I-Algorithm
is	O
sometimes	O
referred	O
to	O
as	O
,	O
or	O
as	O
a	O
branch	O
of	O
,	O
text	B-Algorithm
retrieval	I-Algorithm
.	O
</s>
<s>
Text	B-Algorithm
retrieval	I-Algorithm
is	O
a	O
branch	O
of	O
information	B-Library
retrieval	I-Library
where	O
the	O
information	O
is	O
stored	O
primarily	O
in	O
the	O
form	O
of	O
text	O
.	O
</s>
<s>
Text	O
databases	O
became	O
decentralized	O
thanks	O
to	O
the	O
personal	B-Device
computer	I-Device
.	O
</s>
<s>
Text	B-Algorithm
retrieval	I-Algorithm
is	O
a	O
critical	O
area	O
of	O
study	O
today	O
,	O
since	O
it	O
is	O
the	O
fundamental	O
basis	O
of	O
all	O
internet	B-Application
search	I-Application
engines	I-Application
.	O
</s>
<s>
Document	B-Algorithm
retrieval	I-Algorithm
systems	I-Algorithm
find	O
information	O
to	O
given	O
criteria	O
by	O
matching	O
text	O
records	O
(	O
documents	O
)	O
against	O
user	O
queries	B-Library
,	O
as	O
opposed	O
to	O
expert	B-Application
systems	I-Application
that	O
answer	O
questions	O
by	O
inferring	O
over	O
a	O
logical	O
knowledge	O
database	O
.	O
</s>
<s>
A	O
document	B-Algorithm
retrieval	I-Algorithm
system	I-Algorithm
consists	O
of	O
a	O
database	O
of	O
documents	O
,	O
a	O
classification	O
algorithm	O
to	O
build	O
a	O
full	B-Application
text	I-Application
index	I-Application
,	O
and	O
a	O
user	O
interface	O
to	O
access	O
the	O
database	O
.	O
</s>
<s>
A	O
document	B-Algorithm
retrieval	I-Algorithm
system	I-Algorithm
has	O
two	O
main	O
tasks	O
:	O
</s>
<s>
Evaluate	O
the	O
matching	O
results	O
and	O
sort	O
them	O
according	O
to	O
relevance	O
,	O
using	O
algorithms	O
such	O
as	O
PageRank	B-Algorithm
.	O
</s>
<s>
Internet	B-Application
search	I-Application
engines	I-Application
are	O
classical	O
applications	O
of	O
document	B-Algorithm
retrieval	I-Algorithm
.	O
</s>
<s>
The	O
vast	O
majority	O
of	O
retrieval	O
systems	O
currently	O
in	O
use	O
range	O
from	O
simple	O
Boolean	O
systems	O
through	O
to	O
systems	O
using	O
statistical	O
or	O
natural	B-Language
language	I-Language
processing	I-Language
techniques	O
.	O
</s>
<s>
There	O
are	O
two	O
main	O
classes	O
of	O
indexing	O
schemata	O
for	O
document	B-Algorithm
retrieval	I-Algorithm
systems	I-Algorithm
:	O
form	O
based	O
(	O
or	O
word	O
based	O
)	O
,	O
and	O
content	O
based	O
indexing	O
.	O
</s>
<s>
The	O
document	B-Algorithm
classification	I-Algorithm
scheme	O
(	O
or	O
indexing	B-Application
algorithm	I-Application
)	O
in	O
use	O
determines	O
the	O
nature	O
of	O
the	O
document	B-Algorithm
retrieval	I-Algorithm
system	I-Algorithm
.	O
</s>
<s>
Form	O
based	O
document	B-Algorithm
retrieval	I-Algorithm
addresses	O
the	O
exact	O
syntactic	O
properties	O
of	O
a	O
text	O
,	O
comparable	O
to	O
substring	O
matching	O
in	O
string	O
searches	O
.	O
</s>
<s>
A	O
suffix	B-Architecture
tree	I-Architecture
algorithm	O
is	O
an	O
example	O
for	O
form	O
based	O
indexing	O
.	O
</s>
<s>
The	O
content	O
based	O
approach	O
exploits	O
semantic	O
connections	O
between	O
documents	O
and	O
parts	O
thereof	O
,	O
and	O
semantic	O
connections	O
between	O
queries	B-Library
and	O
documents	O
.	O
</s>
<s>
Most	O
content	O
based	O
document	B-Algorithm
retrieval	I-Algorithm
systems	I-Algorithm
use	O
an	O
inverted	B-Algorithm
index	I-Algorithm
algorithm	O
.	O
</s>
<s>
A	O
signature	O
file	O
is	O
a	O
technique	O
that	O
creates	O
a	O
quick	O
and	O
dirty	O
filter	O
,	O
for	O
example	O
a	O
Bloom	B-Algorithm
filter	I-Algorithm
,	O
that	O
will	O
keep	O
all	O
the	O
documents	O
that	O
match	O
to	O
the	O
query	B-Library
and	O
hopefully	O
a	O
few	O
ones	O
that	O
do	O
not	O
.	O
</s>
<s>
Since	O
in	O
most	O
cases	O
this	O
structure	O
is	O
inferior	O
to	O
inverted	B-Algorithm
files	I-Algorithm
in	O
terms	O
of	O
speed	O
,	O
size	O
and	O
functionality	O
,	O
it	O
is	O
not	O
used	O
widely	O
.	O
</s>
<s>
However	O
,	O
with	O
proper	O
parameters	O
it	O
can	O
beat	O
the	O
inverted	B-Algorithm
files	I-Algorithm
in	O
certain	O
environments	O
.	O
</s>
<s>
The	O
PubMed	B-Library
form	O
interface	O
features	O
the	O
"	O
related	O
articles	O
"	O
search	O
which	O
works	O
through	O
a	O
comparison	O
of	O
words	O
from	O
the	O
documents	O
 '	O
title	O
,	O
abstract	O
,	O
and	O
MeSH	O
terms	O
using	O
a	O
word-weighted	O
algorithm	O
.	O
</s>
