I read today in an email that "75% of what we see on a search engine
is either non proven information or not factual". Has there been any
general studies from credible sources that have attempted to measure
the percentage of factual information on the Internet in any way?

The short answer to your question is that NO ONE is able to measure
how much factual information is on the internet, what is classed as
factual information.

If I make a post on a website that says I am currently writing a post
on this website, does that count as factual information? If I post
that Robert Maynard killed Blackbeard that would definitely be counted
as factual information, but even then the actual names of Blackbeard
are not 100% known.

This website goes to address how to evaluate what is factual information
http://www.library.jhu.edu/researchhelp/general/evaluating/

http://www.forbes.com/2003/01/31/cx_da_0131topnews.html
This article goes some way to trying to find out what % of internet
users think that the internet provides accurate information.

http://www.digitalcenter.org/pages/site_content.asp?intGlobalId=20
This is the research write up document for the question "what % of
Internet users believe the information they find online is accurate".
http://www.digitalcenter.org/downloads/World_Internet_Project.doc

I feel this text is the best answer you will get to the question of
reliable information on the Internet

"The authors of the survey write that the number of users who believe
in the reliability of information on the Internet "continues to
decline." But they have only conducted the survey for three years.
Their data show that, between 2000 and 2001, what might be called the
believability/reliability index grew from 55% to the high-water mark
58% in 2001. What causes the believability index to fluctuate is
unknown."

The following sections give you graphs with relation to the
reliability of the information on the internet
http://www.stateofthenewsmedia.org/chartland.asp?id=119&ct=&dir=&sort=
http://www.stateofthenewsmedia.org/chartland.asp?id=127&ct=col&dir=&sort=&col1_box=1&col2_box=1&col3_box=1
http://www.stateofthenewsmedia.org/chartland.asp?id=129&ct=col&dir=&sort=&col4_box=1
http://www.stateofthenewsmedia.org/chartland.asp?id=118&ct=&dir=&sort=

From this you can see that most people on the internet believe that at
least half to - most of the information on the internet is true.

I know this does not directly answer your question but it is as close
to an answer as you are liable to find. The internet is so vastly
large, ever changing with new pages coming online and old pages
disappearing that is is impossible to guestimate how many web pages
are available on the internet, let alone guessing the % of those
available that have truthful information on them.

I hope this answers your question as best as can be expected, I know
it is not the definitive answer you were hoping for, but as that
answer is not available i hope this serves as a satisfactory
alternative.

--Keystroke-ga

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Clarification of Answer by keystroke-ga on 01 Aug 2006 03:12 PDT

I would also like to say that your initial question stated that you
heard that 75% of the information available online was true.

This statement "From this you can see that most people on the internet
believe that at least half to - most of the information on the
internet is true" would suggest to me that anywhere from 50% to 80% of
the information is perceived to be truthful information.

--Keystroke-ga

I think this is a good answer to my question.

If you include minor errors, then nearly all web sites are inacurate.
Perhaps 10% of the sentences and sentence framents are incorrect or
subject to reasonable doubt. 75% unreliable may be a reasonable
estimate for average web surfers. answers.google.com  of course is
much more reliable than most web services.   Neil