I came across this on the Analytics blog today. Google are developing browser plugins that will allow users to opt out of analytics data collection.
I think it's going to mess with peoples traffic reports, and possibly be overused by users, who are unnecessarily fearful of scary "data collection". It could be like disabling cookies in the past.
That's true, but because GA has never tracked users without JS a site owner could compare reports over time and be comparing apples with the same. If this plugin gets much use then a lot of users with JS enabled could drop out of stats they were previously included in and produce some skewed results.
e.g did my traffic drop 10% compared to this time last year, or did 10% of my visitors install the plugin?
That's where stats based on your server logs will help you to work out the answer.
Since each of the different stats programs reports things in a different way you can get a clearer picture overall by comparing stats from multiple sources. If both Analytics and Awstats show your visitors dropped 10% then they dropped 10% but if Analytics shows the drop and Awstats doesn't then those visitors installed the plugin.
Doesn't help those who use analytics because their free hosting provider doesn't give them access to the server logs though.