It’s been nearly two years since I wrote about using Google’s CDN to host jQuery on your public-facing sites. In that post, I recommended it due to three primary benefits that public CDNs offer: decreased latency, increased parallelism, and improved caching. Though the post has been overwhelmingly well-received, concerns have been raised as to whether or not the likelihood of better caching is truly very significant. Since the efficacy of that benefit depends entirely on how many other sites are using the same CDN, it takes quite a bit of research to make an objective case either way. I’ve never been happy about responding with vague answers. Caching probability is a valid concern and deserves to be taken seriously. So, I decided to cobble together an HTTP crawler, analyze 200,000 of the most popular sites on the Internet, and determine how many of those are referencing jQuery on Google’s public CDN.