I´ve used the CTR and this is in fact the main component of Quality Score, about 65% in fact. What about the other 35%?

About 25% is the keyword´s relevancy. Now, a keyword is either relevant or it´s not. The analogy is that you are either pregnant or you´re not. You can´t be just a little bit pregnant. The same for your keyword being relevant.

Adwords seems to want to see the keyword on your landing page in order to determine its relevancy. It does use what is called latency. In other words, if you sell dog food, the system usually knows that the words canine and puppy are equivalent to dog. You would therefore not likely get hit with poor relevancy if you used puppy food as a keyword even if that exact phrase doesn´t appear on your page.

It also seems to help to have the keyword in your ad although to a lesser extent. Keyword relevancy is guided more by comparing it to what is on your landing page.

Saying that the words must be on your landing page is probably not strictly correct either. While not yet confirmed, Google seems to figure out a theme of your landing page. If the theme matches with the theme of the keyword (more likely the theme of the keywords in your group), you will have good relevancy.

That is likely why there are many advertisers complaining that the keyword is all over their landing page yet getting a poor relevancy score. I´ve experienced this myself. I believe that if the page has a lot of text, Google may have figured out different themes (I use the plural since it likely figures out more than one) which doesn´t match the keywords´ theme. Having a long sales page may just confuse the system. A page can have more than one theme, maybe a maximum of three or four. If there are more, it may decide on one, drop the others based on what it thinks is the main one which may not be the real main theme you intend for that page.

This doesn´t happen often. If you have relevancy, which you should and is fairly easy to get, you get about 2.5 points on your QS. If not, the most you´ll get is a seven but probably only a four or five.

The other 10% of the score is the landing page.

Google has said that the page´s loading time is a factor. If your page loads too slowly, you´ll get your QS reduced. Note that since it is only 10% of the overall score, even a slow loading page can still get a QS of nine. This component by itself cannot bring your QS down to a few points.

However, this component of the QS also considers your page´s quality. Again, like the keyword relevancy portion, this is a go/no-go proposition. In this case however, if your page is deemed of poor quality, it affects the whole QS, not just this small 10%, and is automatically brings it down to a value of one. It might as well be zero because a QS of one is the off switch preventing your ads from ever showing.

Now a lot has been said about landing page quality. Most people see the term and think what Google mean by quality is how the page looks. Nothing is further from the truth.

If you have keyword relevancy, comply with the policies (a "quality" landing page), both of these are easily achievable, what is left is the CTR portion of the QS. That´s why I associate QS with CTR. That´s all you have to worry about, getting that click rate up as high as you can (remember, no matter the position you´re at as explained previously) and your QS will go up. You do this by creating better ads. So you see, Quality Score is not that mysterious.