Google Analytics is a service offered by Google that generates detailed statistics about a website's traffic and traffic sources and measures conversions and sales. It's the most widely used website statistics service.

Google Analytics can track visitors from all referrers, including search engines and social networks, direct visits and referring sites. It also tracks display advertising, pay-per-click networks, email marketing, and digital collateral such as links within PDF documents.

Google Analytics' approach is to show high-level, dashboard-type data for the casual user, and more in-depth data further into the report set. Google Analytics analysis can identify poorly performing pages with techniques such as funnel visualization, where visitors came from referrers, how long they stayed and their geographical position. It also provides more advanced features, including custom visitor segmentation.

However, many ad filtering programs and extensions (such as Firefox's adblocks and NoScripts) can block the Google Analytics Tracking Code. This prevents some traffic and users from being tracked, and leads to holes in the collected data. Also, privacy networks like tor will mask the user's actual location and present inaccurate geographical data. Some users do not have JavaScript-enabled/capable browsers or turn this feature off. However, these limitations are considered small—affecting only a small percentage of visits.

The largest potential impact on data accuracy comes from users deleting or blocking Google Analytics cookies.Without cookies being set, Google Analytics cannot collect data. Any individual web user can block or delete cookies resulting in the data loss of those visits for Google Analytics users. Website owners can encourage users not to disable cookies, for example, by making visitors more comfortable using the site through posting a privacy policy.

These limitations affect the majority of web analytics tools which use page tags (usually javascript programs) embedded in web pages to collect visitor data, store it in cookies on the visitor's computer, and transmit it to a remote database by pretending to load a tiny graphic beacon.

Another limitation of Google Analytics for large websites is the use of sampling in the generation of many of its reports. To reduce the load on their servers and to provide users with a relatively quick response for their query, Google Analytics limits reports to 500,000 randomly sampled visits at the profile level for its calculations. While margins of error are indicated for the visits metric, margins of error are not provided for any other metrics in the Google Analytics reports. For small segments of data, the margin of error can be very large.