* Not sure if this is the right place..

But I've been asked by a client to add Google Analytics to every page of his enormous website...

Has anyone got a clear idea as to how accurate it is, if at all?

On a site that I did test it, it didn't match up with the stats provided by the hosting company.

Which is wrong and which is right then?

I am using Google Analytics and am happy with what I derive from reports. I also run ClickTracks and WebTrends less often but honestly I cannot answer your question.

Information provided by such web analysis tools like Google Analytics, ClickTracks or WebTrends are based on their own interpretation of raw data. Therefore, they are not supposed to be compared with each other. I won't be surprised if they don't match. The discrepancy don't mean one is less accurate than the other. To me it's like how each of them define a certain metric.

If you start using one stats package it's good to focus on that tool. You can always look up to other tools when certain features are not provided or is less comprehensive.

I'm still not sold on it's accuracy. For some odd reason our page views in Analytics is now higher than our IIS logs. I'm not sure how in the world that can happen?!

One explanation as to why page views may be higher in Analytics than your IIS log files is page caching.

When a page is cached at a service provider or web caching location, for speed of delivery, your IIS logs will not show when that page is loaded. Unless you prohibit page caching.

With Google Analytics you place a snippet of code(bug) on each page you want to track. That code calls Google every time that page is loaded, even if from a cached site, or your desktop and says that page was viewed.

Google Analytics, WebTrends on Demand (or whatever its called now), HitBox etc use this kind of tracking.

Most web service providers use a hosted solution, WebTrends, ClickTracks, Urchin (yes you can still get it) that use your log files.

Log file analysis can track web crawling spiders, bugging a site cannot.
Bugging a site will track more page views than log files.

I know that WebTrends offers a mixture of both, if you purchase the right package. And if you must know every little detail of what's going on that's the best way to go.

For the small to mid sized site I'd have to recommend Google Analytics, that is if you don't mind giving Google access to just about everything that happens on your site.