Tracking Code Overview

Google Analytics works by the inclusion of a block of JavaScript code on pages in your website. When visitors to your website view a page, this JavaScript code references a JavaScript file which then executes the tracking operation for Analytics. The tracking operation retrieves data about the page request through various means and sends this information to the Analytics server via a list of parameters attached to a single-pixel image request.

Because your website configuration and reporting needs might differ from a standard setup, it's a good idea to understand the general tracking process to ensure that your reports deliver data as you expect. In this way, you can decide how to configure Analytics tracking to best suit your own website.

The rest of this document covers:

1. How Does Google Analytics Collect Data?
   1. How the Tracking Code Works
   2. How GIF Requests are Classified
   3. GIF Request Parameters

How Does Google Analytics Collect Data?

The data that Google Analytics uses to provide all the information in your reports comes from these sources:

• The HTTP request of the visitor
• Browser/system information
• First-party cookies

The HTTP request for any web page contains details about the browser and the computer making the request, such as the hostname, the browser type, referrer, and language. In addition, the DOM of most browsers provides access to more detailed browser and system information, such as Java and Flash support and screen resolution. Analytics uses this information in constructing reports like the Map Overlay, Browser, and Referring Sites reports. Analytics also sets and reads first-party cookies on your visitors' browsers in order to obtain visitor session and any ad campaign information from the page request. When all this information is collected, it is sent to the Analytics servers in the form of a long list of parameters attached to a single-pixel GIF image request.

The data contained in the GIF request is the data sent to the Google Analytics servers, which then gets processed and ends up in your reports. Here is an example of only a portion of a GIF request:

http://www.google-analytics.com/__utm.gif?utmwv=4&utmn=769876874&utmhn=example.com&utmcs=ISO-8859-1&utmsr=1280x1024&utmsc=32-bit&utmul=en-us&utmje=1&utmfl=9.0%20%20r115&utmcn=1&utmdt=GATC012%20setting%20variables&utmhid=2059107202&utmr=0&utmp=/auto/GATC012.html?utm_source=www.gatc012.org&utm_campaign=campaign+gatc012&utm_term=keywords+gatc012&utm_content=content+gatc012&utm_medium=medium+gatc012&utmac=UA-30138-1&utmcc=__utma%3D97315849.1774621898.1207701397.1207701397.1207701397.1%3B...  

For more information on the data contained in a GIF request, see the section "GIF Request Parameters" in the Troubleshooting Guide.

How the Tracking Code Works

In general, the Google Analytics Tracking Code (GATC) retrieves web page data as follows:

1. A browser requests a web page that contains the tracking code.
   
2. A JavaScript Array named _gaq is created and tracking commands are pushed onto the array.
3. A <script> element is created and enabled for asynchronous loading (loading in the background).
4. The ga.js tracking code is fetched, with the appropriate protocol automatically detected. Once the code is fetched and loaded, the commands on the _gaq array are executed and the array is transformed into a tracking object. Subsequent tracking calls are made directly to Google Analytics.
5. Loads the script element to the DOM.
6. After the tracking code collects data, the GIF request is sent to the Analytics database for logging and post-processing.

How GIF Requests Are Classified

A GIF request is sent to the Analytics servers in the following cases and classified according to the table below. In each of these cases, the GIF request is identified by type in the utmt parameter. In addition, the type of the request also determines which data is sent to the Analytics servers. For example, transaction and item data is only sent to the Analytics servers when a purchase is made. Visitor, page, and system information is only sent when an event is recorded or when a page loads, and the user-defined value is only sent when the _setVar method is called.

Requests classified as interaction requests will impact the bounce rate calculations for your page or site. Bounce rate is referred to as a single-page visit to your site, but is strictly defined as a single interaction request during a user session. For this reason, a bounce rate for a page is also affected by ecommerce transactions and event tracking requests. This is because these features co-exist with page tracking and, when they are triggered, they result in additional interaction requests to the Analytics servers.

The GIF Request Parameters

The GIF request is quite long. Here is an example of only a part of a GIF request:

http://www.google-analytics.com/__utm.gif?utmwv=4&utmn=769876874&utmhn=example.com&utmcs=ISO-8859-1&utmsr=1280x1024&utmsc=32-bit&utmul=en-us&utmje=1&utmfl=9.0%20%20r115&utmcn=1&utmdt=GATC012%20setting%20variables&utmhid=2059107202&utmr=0&utmp=/auto/GATC012.html?utm_source=www.gatc012.org&utm_campaign=campaign+gatc012&utm_term=keywords+gatc012&utm_content=content+gatc012&utm_medium=medium+gatc012&utmac=UA-30138-1&utmcc=__utma%3D97315849.1774621898.1207701397.1207701397.1207701397.1%3B...   

This table contains a listing of the many of the parameters passed in via the GIF Request. Not all parameters are passed in with every execution of the tracking code, since some apply only to certain conditions, such as campaign referrals or shopping carts. When using this reference, keep in mind that you will be looking for those variables that most commonly apply to the page/request you are investigating.