Answer: Your Google Quality Score (or its Yahoo! equivalent, the Quality Index) helps determine your minimum keyword bids, how high your pay-per-click ads rank - and whether your ads are eligible to appear at all. In general, low scores for individual keywords mean you’ll have to pay more than your competitors for the same rank.

Google does track the historical performance of your account, based on the click-through rate of all your ads and keywords over time, but it does not assign an overall quality score for your account – at least not where you can see it. You can’t look it up, and you certainly can’t comfort yourself by saying, “My quality score is a 5.6 and the average for all companies is a 5.2, so I must be doing ok.”

What you can do is look at individual ad groups to see which keywords are ranked "poor," "ok" or "great" – but only if you change Google's default display settings first.

Now let's get to the more important question – how can you improve your quality scores?

Both Google and Yahoo! use top-secret formulas to calculate your quality scores, but the main ingredients are your click-through rates from the PPC ads themselves, the relevance of the ads to the keywords, the quality of your landing pages and your overall historic performance. (To learn more, read the guidelines for Google's Quality Score and Yahoo!'s Quality Index.)

The key to a better score is having each keyword match each ad, and having each ad tightly match each landing page. For example, let's say you have an "ice cream" ad group that includes several keyword variations, such as "chocolate ice cream," "ice cream sundae" and "retail ice cream." Your ad must apply to every keyword in this group and should have the words "ice cream" in the headline. Likewise, the words "ice cream" should also appear in both the title and headline of your landing page.

When it comes to your landing page, quality matters – a lot. In addition to assessing how relevant the landing page is to the keyword in question, Google also evaluates a host of other factors, such as content originality, the number of links to other resources, the presence or absence of a privacy-policy link, load time and the use of clean, standards-based XHTML code, to name a few.

You should systematically test what does and doesn't work by conducting controlled experiments with a single ad group. Create two different ads, changing only one element at a time: a different headline in the ad, a different headline on the landing page or a different landing page title. Make sure you specify that the two ads are to get equal screen time. (In Google, you do this on the "Edit Campaign Settings" screen, by choosing the "Rotate: Show ads more evenly" radio button under "Scheduling and Serving.")

Let the two test ads run until one of them gets about 25 clicks. If one version gets 25 clicks and the other only gets 10, you can be certain that the change you've implemented may be worth doing across additional ads or landing pages. If the score is less conclusive – 25 clicks to 22 clicks, for instance – the change may not be worth the effort.

So, let's assume you've made an improvement to either your ad or your landing page. It's easy to see the improvement if your quality score jumps from "poor" to "ok" for that keyword. But what if the needle is stuck at "ok," or you're already "great" but want to be even better? You can still measure improvement by keeping track of what really matters in the first place: your minimum CPC bid and your page ranking. If your minimum bid is going down or your page ranking is going up, your changes are working. If not, keep testing and making changes until your results improve.