<s>
Keyword	B-General_Concept
extraction	I-General_Concept
is	O
tasked	O
with	O
the	O
automatic	O
identification	O
of	O
terms	O
that	O
best	O
describe	O
the	O
subject	O
of	O
a	O
document	O
.	O
</s>
<s>
The	O
task	O
of	O
keyword	B-General_Concept
extraction	I-General_Concept
is	O
an	O
important	O
problem	O
in	O
text	B-Algorithm
mining	I-Algorithm
,	O
information	B-General_Concept
extraction	I-General_Concept
,	O
information	B-Library
retrieval	I-Library
and	O
natural	B-Language
language	I-Language
processing	I-Language
(	O
NLP	B-Language
)	O
.	O
</s>
<s>
keyword	B-General_Concept
extraction	I-General_Concept
(	O
keywords	O
are	O
chosen	O
from	O
words	O
that	O
are	O
explicitly	O
mentioned	O
in	O
original	O
text	O
)	O
.	O
</s>
<s>
Methods	O
for	O
automatic	O
keyword	B-General_Concept
extraction	I-General_Concept
can	O
be	O
supervised	O
,	O
semi-supervised	O
,	O
or	O
unsupervised	O
.	O
</s>
<s>
Unsupervised	O
methods	O
can	O
be	O
further	O
divided	O
into	O
simple	O
statistics	O
,	O
linguistics	O
or	O
graph-based	O
,	O
or	O
ensemble	B-Algorithm
methods	I-Algorithm
that	O
combine	O
some	O
or	O
most	O
of	O
these	O
methods	O
.	O
</s>
