Str82u - 4:02 pm on Feb 23, 2013 (gmt 0)
That's to say that you have to take the stats (statistics) from AdSense as the only source of information unless you have an additional tracking on the ad code (we inject ads on half our sites so I can see how many times the file with the code gets requested and can match that with the statistics from AdSense). That doesn't do any good for tracking CTR but there are solutions for that too if you look for them.
I really don't know what you're talking about when you say "stats are pure speculation"
Bottom line is that only Google knows the true numbers so if there's a drop in CTR there are only a few reasons:
1. People just stopped clicking.
2. Ads changed content/behavior and don't attract clicks.
3. Google's cheating you
4. Google's filtering your clicks
5. Someone is hijacking your clicks
The most likely of those is usually #2 or #4, as someone else pointed out, Google has too much at stake to cheat anyone like #3. For #5 you can "Fetch as Googlebot" and look at a few pages source to see if your ad code is yours but that's not a thorough method of detecting hijacking or hacking.
Follow @netmeg's advice and take a few minutes to contact AdSense support or post on the official forum [productforums.google.com...] - the longer you delay the longer it will take for answers.