I haven't posted about Sitemeter for a while, even that they are most probably the largest web analytics solution out there. Here some stuff you should know about the way they measure Visit: " SiteMeter does not currently track Unique Visits/Visitors. Our measurement is defined as a Visit. The basis of this measurement is calculated on a 30 minute time period, from the last page viewed (i.e. last activity seen).
As you are no doubt aware, a Unique Visitor is measured using a cookie (or other similar user hosted ID tag). This method permits one to measure hourly, daily, weekly, etc. unique visitors. You could even measure the number of unique over an entire year - which by the way will be an option for SiteMeter members in an upcoming new phase release. Of course there are limitations to this form of measuring traffic, not the least of which is people who disable cookies. "