That’s the question that I was asked recently by one of the co-workers. It is simple and not so simple at the same time. It really depends on what you are looking for, what is the acceptable accuracy, and what is that you are comparing Google Analytics with.
For example, if you compare the numbers from your Google Analytics reports to the summaries of the web server logs, you’ll probably find that Google Analytics reports lower numbers. Almost like not everything is recorded. Which is true because Google Analytics is using JavaScript to track your visitors. Server logs record all hits to your web server, but the information in logs is very limited – it won’t be enough for anything but very basic tracking.
How much will numbers differ? Here is what Google Analytics blog has to say:
Google Analytics uses JavaScript tags to collect data. This industry-standard method yields reliable trends and a high degree of precision, but it’s not perfect. Most of the time, if you are noticing data discrepancies greater than 10%, it’s due to an installation issue. Common problems include JavaScript errors, redirects, untagged pages and slow client-side load times.
Having used Google Analytics on a number of sites over a number of years, I’d say that that is just about right.