AdWords implements a minimum CTR in order to (a) avoid serving stale ads; (b) make you work harder on fine-tuning your ads and keywords. Here's a story on how I got bitten by the minimum CTR and how I managed to fine-tune my keywords.
I recently decided to use Google AdWords to promote my book. It is aimed at a relatively small market and I was hoping to use AdWords to create maximum exposure.
It seemed simple. I wrote an ad that announced my book, and used two words from the title as keywords. google began serving ads immediately but
the CTR was close to 0.0% even tough a lot of people clikcked on the link and ordered the book.
The next morning I received mail informing me that the delivery of my ad has been suspended and that I should make changes to my keywords. One of these keywords was the main keyword, people clicked on the ad, but they did not generate the required minimum CTR. With the main keyword suspended, I was trapped. However, after a few hours of experimenting and watching results, I founf that it is possible to achieve a balance between the number of ads served and the CTR they generate if I put keyrdws or groups of keywors inside parentheses or sqare brackets, e.g. "jacek" or [jacek] instead of plai: jacek.