When you are evaluating your website for its effectiveness and appeal (and you should do this from time to time, just like PDs critically examine their on-air product), the thought process that many people undertake is to look at the site with their own eyes and then to imagine how the site appears to both first-time and then returning visitors. Is the site easy to understand? Is it simple to navigate around? Is it visually appealing?

There is a complication to this. What if the website your visitors are seeing isn't the website you see every day? Sure, everyone's experience is subjective and thus open to variation - but your site actually appears objectively different to your visitors depending on what Operating System they are using, what plugins they have installed, which browser they're using, what type of monitor they are viewing, etc...