PageRank:
The Background
Google's
PageRank (PR) is one of the most sought after, and yet
misunderstood web page attributes. PageRank, named after
one of
the founders of the Google search engine, Larry Page,
was the
innovative foundation that the Google search engine
was built on.
The
theory was that a link from one web page to a web page
of
another site was in essence a vote for that page. The
reasoning
was that webmasters would only link to pages that they
thought
were interesting and of value to their viewers. Google
used the
number of inbound links (IBL) to a page to judge the
importance
and relevance of that page, and based on this calculation,
and
other factors, decided where to place that page on the
search
results page (SERP).
They
devised a scale of measurement for PageRank from 1 to
10.
Then for the information of webmasters and interested
people
they produced a toolbar that can be deployed in Internet
Explorer
that indicates the PageRank value of any page being
viewed in
the browser. These values have become known as PR0 to
PR10.
Since
PR values are a result of IBLs, Google decided to give
them their own name and refers to inbound links as backlinks.
As part of the toolbar there is a quick lookup of the
number of
backlinks that Google reports for the page that is currently
being viewed in the browser. This search can also be
done
without the aid of the toolbar by simply typing
"link:http://www.yourURL.com" into the Google
search box.
The
one trick to this link search is that Google does not
display all backlinks. At one time it was thought that
they only
listed pages with a value of PR4 or greater. Today however,
you
will find backlinks reported from pages of lower PR
values. So,
at best, Google's backlink search seems to present some
sample
of pages linking to the site. Suffice it to say that
this search
is not a reliable measure of all IBLs to a page.
How
is PageRank Calculated?
In
simplest terms PR is calculated by the sharing of PR
from
all the IBL links to your page. This is not strictly
accurate
because Google also uses the internal links within a
site in
the calculation of PR. Each link to a page carries with
it and
passes PR value to the target page. The PR points or
value
passed depend on the PR value of the page they come
from, and
the total outbound links from the page. It is generally
agreed
that a page will only pass about 85% of its value to
the page it
links to. So a PR5 page with a single outbound link
will pass
85% of the value of a PR5 page to the page it links
to.
But
virtually no page has only a single link -- remember
internal links are also used in the total outbound link
count -- so the value passed to any page is 85% of the
PR,
divided by the total number of outbound links.
The
question now becomes what is the PR point value of the
different PR levels. Most observers believe that the
relationship
between PR levels is logarithmic rather than linear.
In other
words PR5 is not worth 25% more than a PR4, but may
be worth
4 to 6 times more.
It
is also understood that a PR value is not a single number,
but is in fact a range of values. So not all PR6s are
equal. As
the chart below shows a PR6 may be just on the upper
boundary of
a PR5 or it may be just short of the entry point for
a PR7.
The
chart that follows shows the range for each PR value.
It
also shows how much PR value or PR points a page with
50 outbound
links will pass depending on of its own PR rank. From
this I have
calculated the number of links required from each value
of
PageRank necessary for a page to attain a desired page
rank.
Here
is a PageRank Calculation Chart:
(http://www.sitepronews.com/pagerank.html).
The
Assumptions And The Mathematics:
For
those who are interested I have used logarithmic values
of
base 5.5. In other words the value range for a PR1 lies
between
5.5 to the power of 1 and 5.5 to the power of 1.99,
and PR2 lies
between the value of 5.5 to the power of 2 and 5.5 to
the power
of 2.99 etc. The rest of the chart is fairly straight
forward.
It assumes that there are 50 links per page and that
85% of the
PR value is passed to the recipient page.
The
number of links required to attain any ranking is based
on
the median value of the donor page and the entry threshold
of
the desired PR value. In other words to achieve a PR5
you need
5,033 points and the average points available from a
PR6 page
with 50 links is 1507.
The
chart was calculated with an Excel spreadsheet and it
can
be downloaded if anyone wants to play with the calculations
and
assumptions. It might be interesting to work with a
different
base number for the logarithmic calculation. And it
is also
interesting to see the impact of more or less outbound
links
from a page.
Are
These Findings Valid?
Nobody
knows for sure how Google calculates PR. I have shown
this chart to a number of knowledgeable people and they
have all
agreed that my calculations look reasonable. One SEO
guru from a
major firm said the results were very similar to independent
research that his firm had conducted.
So
take it or leave it. It is probably a fair reflection
of how
PR is passed and accumulated.
The
lesson that can be drawn from this explanation and chart
is
that if you want to increase your PR you need a few
links from
pages with equivalent or higher PR, or a great many
links from
sites with lower PR.
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Bob Wakfer is the owner of Computer Partners and a long
time
student and practitioner of SEO. Computer Partners offers
web
site design, hosting and search engine optimization
for its
clients. If you would like to discuss any of these services
with Bob you can email him at bob@compar.com.
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