I'm not sure about his article and his formula, but it is already a debate in the web analytics industry. http://www.omniture.com/blog/ Even using cookies it's nearly impossible to get correct unique visitor counts and that is why the industry is moving more towards unique visits, because a visit is a visit, it doesn't matter who the visitor was... The only way to really measure how far off visitor data could be is comparing unique customers (cusomter id) to the number of unique visitors they create (the customer id coming from a login). That way they could see the affects of multiple customers on multiple machines and browsers and also see the affects of multiple customers on a single machine and browser.