3-4% is normal for interstitial ads I get same click thru rates on 1000's of apps with admob combined with banners im at 1 percent.
Admob did the best in dec 2015
It's very strange. I didn't get any invalid traffic in last Dec usually it takes 1-5% of my total revenue.
I also got $250 invalid traffic between 26-28 dec.
i am new and i start my app one the 1st of new year
Happy to have Leo Safeguard V3 in my Android