IP – Unique visitors can be measured using a number of methodologies. Traditional Log analysis used IP addresses to identify unique users however this approach over reports people using dialup connections and under reports people using corporate gateways to access the internet.
Cookie – The most common method of evaluating the number of unique visitors to a site is by giving users browsers a cookie that can be used to identify that person as unique. This system is more accurate than IP but there is still the issue of people whole block or delete cookies and also people who use more than one PC to access the internet (business and home) and people who share a login (family PC). However this is the industry standard for reporting unique visitors and Red Sheriffs particular approach to unique cookie tracking is recognized by OPG (Online Publishers Group)
Identified – The only true measure of unique people is to require a login to access the site or place and order. This can be supported by cookies to store login names between sections. This approach will always underreport but does provide the number of unique people who actively interact with you site.