Measuring website traffic

Peter Edmonston of The New York Times has an interesting article about how Forbes.com apparently exaggerates the number of
unique visitors to the website.

During the dot-bomb times, advertised measures of website activities were bizarrely
corrupt. A few years ago NASA (and its then-director) reported that their website during a
Mars visit had more “hits”
than there are people on Earth.

I gather that “unique visitors” and “pageviews” provide some reasonable measures of
website traffic. But the time period for measuring “unique” should be specified, such as
unique visitors each day. (Is that what “unique” usually means?) For
smaller
websites, “pageviews” are substantially inflated by search engine crawlers; our site often
gets 1,400
pageviews per day from Google alone and a good many spurious pageviews from
spammers, email-address harvesters, and the like. It’s just computers talking to other
computers,
not pageviews by a human.

Traffic measures can also be generated by user polling. I understand that Alexa provides
estimates of
traffic counts for many websites, but it fails to document that information concerning its
biased
methodology, which
only polls nonrandomly selected users of Microsoft Windows.

Can our Kindly Contributors provide some authoritative information or links on this matter
(especially from the point of view of data quality and data integrity)?