Generally speaking web analytics programs such as Google Analytics report lower numbers than ad servers. This discrepancy is caused by several factors:
2 – Tracking methods: There are two main methods of tracking activity: cookie-based and IP + User Agent. Google Analytics is a cookie-based analytics program. As such, it relies on a browser setting the cookie. If cookies are disabled, this type of program will not count the visit. This would exclude, for example, hits from a robot or spider. Ad servers use IP + User Agent tracking. This tracking method typically uses log file analysis. This may report higher numbers than reported by cookie-based tracking because of dynamically assigned IP addresses and spider and robot visits.
3 – 1st party vs. 3rd party cookies: Even among cookie-based tracking solutions, there is a difference between 1st party and 3rd party cookies. Because 3rd party cookies are set by a source other than the website being visited, they’re often blocked by browsers and security software. Google Analytics uses 1st party cookies.
4 – Reporting Limits: Google Analytics limits a site visit per user to one time every 30 minutes. Ad servers, by comparison, would not filter such behavior, but would recognize the fact that it is a unique visitor (using a cookie) coming to the page more than once. So impressions would be counted separately from unique impressions. Google would simply filter the multiple impressions out and give the unique impression.
5 – Comparing Apples to Oranges – Clicks vs. Visits: There is an important distinction between clicks recorded by the ad server and the visits recorded by Google Analytics. The clicks column in a campaign report refers to how many time the advertisement was clicked by visitors while visits in a Google report indicates the number of unique sessions initiated by visitors. A visitor may click an ad multiple times. When one person clicks on one advertisement multiple times in the same session, the ad server will record multiple clicks while Google Analytics recognizes the separate page views as one visit. This is a common behavior among visitors engaging in comparison shopping. A user may click on an ad, and then later, during a different session, return directly to the site through a bookmark. The referral information from the original visit will be retained in this case, so the one click will result in multiple visits.
6 – Partial Page Load: A visitor may click on an advertisement, but prevent the page from fully loading by navigating to another page or by pressing their browser’s Stop button. In this case, Google Analytics’ tracking code is unable to execute and send tracking data to the Google servers. However, the ad servers will still register a click.
7 – Other Reasons Causing Discrepancies between Google Analytics and the Ad Server: