Where to Place the Tracking Code
The tracking code is designed to read data from your page after the content for the page has finished loading. For this reason, the snippet should be located just before the closing </body> tag for your web page. Once the content for your page has loaded, the tracking code, when executed, reads the content for your page following the Document Object Model (DOM). All information relevant to tracking is then used to establish page information, set/update cookies, and to send the GIF request to the Google Analytics servers.
By placing the script at the end of the page body, you ensure that the tracking code is executed as the last element of the DOM. If a given page load is interrupted for some reason, it is possible that the GIF request for that page view will not be executed. However, should you place the tracking code at the top of the page, any load interruptions might result in incomplete or inaccurate reporting anyhow, since the tracking code relies on page data for its reports.
Additionally, the physical placement of the tracking code call at the bottom of the page is more effective than using an onLoad() function to call the tracking code. If you use onLoad() to execute the tracking code, execution relies on the event model for the browser instead of the DOM. In such a situation, should a remote image fail to load on a page, onLoad() will not be called, whereas the DOM for the page could still load completely.