The problem
I needed to answer this question: How many readers will be affected by the bigger size of the new site design?
Basically, I needed to know how many visitors are using a screen resolution higher or lower than the one I had in mind, which is 1280×800. A high proportion of visitors with a lower resolution than that would mean that I’d better reconsider downsizing the site to, say, a 1024 width, as it’ll impact negatively the usability of the site for them.
Fortunately, Google Analytics has a nice feature that can answer that question as long as you want to deal with regular expressions. And, fortunately again, I’ve done the hard work for you.
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