Estimate of under-reporting of Google Analytics?

Users with cookies, js turned off?

[sublime1]

I searched around on the web, and here and was not very successful at getting an idea of how much GA under-reports traffic. Does anyone know?

We're running some ad campaigns from FB, StumbleUpon and others, all tagged with GA tracking tags. FB reports about 50% more click-throughs than we're seeing in GA. Of course GA is going to under-report for users with cookies or JS disabled, or adblock, etc. But this 50% seems pretty high to me.

Any pointers to reasonably accurate estimates of this would be greatly appreciated.

TomH

[tangor]

Those running with JS off, adblock/noscript, etc. may tend to have more trust in sites OTHER than google? If that is a reasonable assumption, you have your multiplier in hand.

[sublime1]

@tangor --

Sorry if I wasn't clear.

I am looking for data of how much Google Analytics tends to record fewer visits due to users turning off JS, Cookies or doing other things that will prevent tools like Google Analytics from recording visitors.

Thanks!

TomH