<s>
Prediction	B-Algorithm
by	I-Algorithm
partial	I-Algorithm
matching	I-Algorithm
(	O
PPM	O
)	O
is	O
an	O
adaptive	O
statistical	O
data	B-General_Concept
compression	I-General_Concept
technique	O
based	O
on	O
context	O
modeling	O
and	O
prediction	O
.	O
</s>
<s>
PPM	O
models	O
use	O
a	O
set	O
of	O
previous	O
symbols	O
in	O
the	O
uncompressed	B-General_Concept
symbol	O
stream	O
to	O
predict	O
the	O
next	O
symbol	O
in	O
the	O
stream	O
.	O
</s>
<s>
PPM	O
algorithms	O
can	O
also	O
be	O
used	O
to	O
cluster	O
data	O
into	O
predicted	O
groupings	O
in	O
cluster	B-Algorithm
analysis	I-Algorithm
.	O
</s>
<s>
In	O
many	O
compression	B-General_Concept
algorithms	I-General_Concept
,	O
the	O
ranking	O
is	O
equivalent	O
to	O
probability	O
mass	O
function	O
estimation	O
.	O
</s>
<s>
For	O
instance	O
,	O
in	O
arithmetic	B-Algorithm
coding	I-Algorithm
the	O
symbols	O
are	O
ranked	O
by	O
their	O
probabilities	O
to	O
appear	O
after	O
previous	O
symbols	O
,	O
and	O
the	O
whole	O
sequence	O
is	O
compressed	O
into	O
a	O
single	O
fraction	O
that	O
is	O
computed	O
according	O
to	O
these	O
probabilities	O
.	O
</s>
<s>
A	O
variant	O
called	O
PPMd	B-Algorithm
increments	O
the	O
pseudocount	O
of	O
the	O
"	O
never-seen	O
"	O
symbol	O
every	O
time	O
the	O
"	O
never-seen	O
"	O
symbol	O
is	O
used	O
.	O
</s>
<s>
(	O
In	O
other	O
words	O
,	O
PPMd	B-Algorithm
estimates	O
the	O
probability	O
of	O
a	O
new	O
symbol	O
as	O
the	O
ratio	O
of	O
the	O
number	O
of	O
unique	O
symbols	O
to	O
the	O
total	O
number	O
of	O
symbols	O
observed	O
)	O
.	O
</s>
<s>
The	O
actual	O
symbol	O
selection	O
is	O
usually	O
recorded	O
using	O
arithmetic	B-Algorithm
coding	I-Algorithm
,	O
though	O
it	O
is	O
also	O
possible	O
to	O
use	O
Huffman	B-General_Concept
encoding	I-General_Concept
or	O
even	O
some	O
type	O
of	O
dictionary	B-Algorithm
coding	I-Algorithm
technique	O
.	O
</s>
<s>
Software	O
implementations	O
were	O
not	O
popular	O
until	O
the	O
early	O
1990s	O
because	O
PPM	O
algorithms	O
require	O
a	O
significant	O
amount	O
of	O
RAM	B-Architecture
.	O
</s>
<s>
Recent	O
PPM	O
implementations	O
are	O
among	O
the	O
best-performing	O
lossless	B-Algorithm
compression	I-Algorithm
programs	O
for	O
natural	O
language	O
text	O
.	O
</s>
<s>
PPMd	B-Algorithm
is	O
an	O
implementation	O
of	O
PPMII	O
by	O
Dmitry	O
Shkarin	O
.	O
</s>
<s>
It	O
is	O
also	O
used	O
by	O
7-Zip	B-Language
as	O
one	O
of	O
several	O
possible	O
compression	O
methods	O
in	O
the	O
7z	O
file	O
format	O
.	O
</s>
<s>
Attempts	O
to	O
improve	O
PPM	O
algorithms	O
led	O
to	O
the	O
PAQ	B-Application
series	O
of	O
data	B-General_Concept
compression	I-General_Concept
algorithms	I-General_Concept
.	O
</s>
<s>
A	O
PPM	O
algorithm	O
,	O
rather	O
than	O
being	O
used	O
for	O
compression	O
,	O
is	O
used	O
to	O
increase	O
the	O
efficiency	O
of	O
user	O
input	O
in	O
the	O
alternate	O
input	O
method	O
program	O
Dasher	B-Language
.	O
</s>
