<s>
The	O
horizon	B-General_Concept
effect	I-General_Concept
,	O
also	O
known	O
as	O
the	O
horizon	O
problem	O
,	O
is	O
a	O
problem	O
in	O
artificial	B-Application
intelligence	I-Application
whereby	O
,	O
in	O
many	O
games	O
,	O
the	O
number	O
of	O
possible	O
states	O
or	O
positions	O
is	O
immense	O
and	O
computers	O
can	O
only	O
feasibly	O
search	O
a	O
small	O
portion	O
of	O
them	O
,	O
typically	O
a	O
few	O
plies	O
down	O
the	O
game	O
tree	O
.	O
</s>
<s>
When	O
evaluating	O
a	O
large	O
game	O
tree	O
using	O
techniques	O
such	O
as	O
minimax	B-Algorithm
with	O
alpha-beta	B-Algorithm
pruning	I-Algorithm
,	O
search	O
depth	O
is	O
limited	O
for	O
feasibility	O
reasons	O
.	O
</s>
<s>
When	O
a	O
significant	O
change	O
exists	O
just	O
over	O
the	O
horizon	O
of	O
the	O
search	O
depth	O
,	O
the	O
computational	O
device	O
falls	O
victim	O
to	O
the	O
horizon	B-General_Concept
effect	I-General_Concept
.	O
</s>
<s>
He	O
split	O
the	O
effect	O
into	O
two	O
:	O
the	O
Negative	O
Horizon	B-General_Concept
Effect	I-General_Concept
"	O
results	O
in	O
creating	O
diversions	O
which	O
ineffectively	O
delay	O
an	O
unavoidable	O
consequence	O
or	O
make	O
an	O
unachievable	O
one	O
appear	O
achievable.	O
"	O
</s>
<s>
For	O
the	O
"	O
largely	O
overlooked	O
"	O
Positive	O
Horizon	B-General_Concept
Effect	I-General_Concept
,	O
"	O
the	O
program	O
grabs	O
much	O
too	O
soon	O
at	O
a	O
consequence	O
that	O
can	O
be	O
imposed	O
on	O
an	O
opponent	O
at	O
leisure	O
,	O
frequently	O
in	O
a	O
more	O
effective	O
form.	O
"	O
</s>
<s>
Greedy	B-Algorithm
algorithms	I-Algorithm
tend	O
to	O
suffer	O
from	O
the	O
horizon	B-General_Concept
effect	I-General_Concept
.	O
</s>
<s>
The	O
horizon	B-General_Concept
effect	I-General_Concept
can	O
be	O
mitigated	O
by	O
extending	O
the	O
search	O
algorithm	O
with	O
a	O
quiescence	B-General_Concept
search	I-General_Concept
.	O
</s>
<s>
This	O
gives	O
the	O
search	O
algorithm	O
ability	O
to	O
look	O
beyond	O
its	O
horizon	O
for	O
a	O
certain	O
class	O
of	O
moves	O
of	O
major	O
importance	O
to	O
the	O
game	O
state	O
,	O
such	O
as	O
captures	O
in	O
chess	B-Application
.	O
</s>
<s>
Rewriting	O
the	O
evaluation	O
function	O
for	O
leaf	B-Data_Structure
nodes	I-Data_Structure
and/or	O
analyzing	O
more	O
nodes	O
will	O
solve	O
many	O
horizon	B-General_Concept
effect	I-General_Concept
problems	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
chess	B-Application
,	O
assume	O
a	O
situation	O
where	O
the	O
computer	O
only	O
searches	O
the	O
game	O
tree	O
to	O
six	O
plies	O
and	O
from	O
the	O
current	O
position	O
determines	O
that	O
the	O
queen	O
is	O
lost	O
in	O
the	O
sixth	O
ply	O
;	O
and	O
suppose	O
there	O
is	O
a	O
move	O
in	O
the	O
search	O
depth	O
where	O
it	O
may	O
sacrifice	O
a	O
rook	O
,	O
and	O
the	O
loss	O
of	O
the	O
queen	O
is	O
pushed	O
to	O
the	O
eighth	O
ply	O
.	O
</s>
