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Web Site Measurement Hacks by Eric T. Peterson

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Hack #29. Understand Web Bugs

A very important aspect of JavaScript page tagging is the use of a small image request to send information to the data collection device. Often referred to as a "web bug" or "web beacon," these images are usually one-by-one pixel image requests designed to carry data from place to place.

Many consumers are told to fear web bugs and web beacons as some type of spyware that will, if allowed, somehow ruin their computers (and potentially their entire lives!). Fortunately, when used in the context of web measurement, nothing could be further from the truth. Web bugs and beacons are simply a rather unsophisticated way to refer to the image request that many third-party web measurement and tracking systems make to collect data. Still, a Google search for "web bug" yields over 10 million documents, so it's worth understanding how consumers respond to the idea of a "bug" and, just in case someone accuses you of using web bugs, have a suitable response ready.

Really, What Is a "Web Bug"

Again, a "web bug" is a misunderstood term that addresses how third-party tracking applications work and what they're able to track. According to the Wikipedia:

A web bug (also known as a tracking bug, pixel tag, web beacon, or clear gif) is a technique for determining who viewed an HTML-based email message or a web page, when they did so, how many times, how long they kept the message open, etc.

Usually, a web bug is a transparent image or an image in the color of the background ...

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