1. Attention Deficit Web Surfers.
As then-United States Senator Ted Stevens will tell you, the internet is a series of tubes. Tubes filled with water naturally in which Web Surfers play. Some of these people have a hard time letting a webpage fully load before they decide to click onto the next image or page or website, so your analytics tracking code may never load for them (especially if placed too far down in the document or is reacting slowly due to server load). The surfer might also have a slower internet connection which might lead to them requesting the next page before the current one is loaded. Google has had a code snippet available to solve this problem for some time. Click here to read more about it.
2. Special hacks.
This type of surfer is probably very very uncommon. They have edited their hosts file to explicitly block their computer from communicating with the analytics site. There are also browser add-ons available which do essentially the same thing. AdBlock and Ghostery are two examples.
JS can be turned off for a variety of reasons…from paranoid super-users to handicapable people using special browsing tools. Estimates of browsers without JS range from 1-2% to as high as 5%, which could be significant.
4. People who never visit your site but who would if they knew about it.
It seems elementary, but you need to think about all the people in your target demographic who could and would visit, but never get to do so because they’ve never been given a link to your site. Think about where such people hang out on the web and create some targeted advertising to get them to stop by! Tools like Google Ad Planner and Hitwise can aide you in finding them.
5. Cookie Monsters.
These surfers have their browser setup to eat up his browser’s cookies after each window closes or they clear cookies often for personal reasons. This surfer is a fresh face every time he visits your site after a cookie purging and there’s nothing you can do about it.
6. Cookie averse users.
Unlike the Cookie Monster, this surfer doesn’t even have the ability to accept a cookie in the first place. Without a persistent cookie tracking the surfer’s moves, each pageview will look like the start of a new visit (to some analytics engines). Google Analytics won’t track the surfer at all.
7. Anonymous Proxies and Tor
These kinds of visitors are also uncommon, but depending on the reach of your site you may see some of them. They are not trackable in server logs or analytics because they are completely anonymous. The Tor project aims to prevent stalkers and cybercriminals from discovering your physical location through network traffic analysis. Privacy Win! Analytics Fail!
8. Mobile surfers
Tips and Tricks
Another suggestion dealt with using a 1×1 pixel image whose source is a tracking script. These web bugs (beacons) are commonly used by 3rd party ad servers when they can’t serve a banner ad. If you were to setup querystrings for the script to grab when the image is served then you could track what Google misses. This can be setup in the NOSCRIPT section.