How to determine if Google Analytics is accurate

I've got a website and I'm tracking its traffic with Google Analytics. Besides, on some pages, I have a counter which is simply incremented each time the page is requested.

I noticed that most of the time, Google Analytics and my counters disagree (GA roughly indactes only half the number of my counter). So I fetched my web servers logs, and for one specific page I calculated the number of different IP addresses who accessed it. At the end, I have :

• Google Analytics : 966 views
• Counter : 2716 views
• Unique IP addresses : 2462 IPs

So my counter and the weblogs seem to confirm each other. I'm a bit surprised that GA seems wrong (you would suspect Google to be able to give correct analytics ...). Any clues of what is happening here ?

Thanks.