<s>
RSS	B-Protocol
tracking	I-Protocol
is	O
a	O
methodology	O
for	O
tracking	O
RSS	O
feeds	O
.	O
</s>
<s>
The	O
problem	O
with	O
this	O
method	O
is	O
that	O
not	O
all	O
RSS	O
feed	O
aggregators	O
will	O
display	O
images	O
and	O
parse	O
HTML	B-Language
.	O
</s>
<s>
Some	O
aggregators	O
(	O
for	O
example	O
,	O
Bloglines	B-Application
and	O
Google	B-Application
Reader	I-Application
)	O
include	O
a	O
number	O
of	O
unique	O
users	O
on	O
whose	O
behalf	O
the	O
feed	O
is	O
being	O
downloaded	O
in	O
the	O
HTTP	O
request	O
.	O
</s>
<s>
Other	O
readers	O
--	O
such	O
as	O
web	O
browsers	O
--	O
can	O
be	O
counted	O
by	O
noting	O
the	O
number	O
of	O
unique	O
IP	B-Protocol
addresses	I-Protocol
that	O
retrieve	O
the	O
file	O
in	O
a	O
given	O
period	O
.	O
</s>
<s>
This	O
provides	O
an	O
estimate	O
of	O
actual	O
readership	O
,	O
although	O
it	O
is	O
probably	O
higher	O
than	O
the	O
real	O
number	O
because	O
people	O
may	O
sign	O
up	O
for	O
accounts	O
with	O
multiple	O
aggregators	O
and	O
never	O
delete	O
their	O
subscriptions	O
and	O
because	O
they	O
may	O
read	O
the	O
same	O
feeds	O
at	O
different	O
computers	O
,	O
or	O
the	O
same	O
computer	O
may	O
have	O
a	O
different	O
IP	B-Protocol
address	I-Protocol
at	O
different	O
times	O
.	O
</s>
