Those paying for each click on their site want to avoid the clicks of those unlikely to buy what they are selling. Pay per click (PPC) people also want to increase the probability that site visitors who are prospective buyers will buy.

This may be more difficult than it appears. For one thing, some keywords are very popular among PPC bidders. It is difficult to stand out in such crowds.

For another, some products and services are not always driven by an immediate and pressing felt need in the prospective buyer. Educating the consumer and developing trust are needed before any sales take place (or any major sales). If the website and given market cannot be matched with keywords that have a high probability of producing sales, a somewhat lower probability must suffice.

A third difficulty is that some keywords may be commonly used in more than one context with more than one meaning. A person typing and searching for the word "plaque" may want information about an ornament to hang on the wall or a solution to a dental problem.

The person typing in the word "bow" may have in mind the contexts of archery (bow and arrow), stringed instruments (violin bow), ships (front end of a boat), or ornamental ribbons (tied red bow).

Yet again, if a person is looking for "work from home" opportunities, the kind of such opportunities the searcher might consider may vary widely.

To increase the probability that site visitors will buy, the person paying for clicks would also want to bid on more specific and focused terms--long tail keywords. This can be done by adding adjectives or qualifying words and by eliminating certain possibilities. The broad match "bow" could be qualified in an exact or phrase match to "compound bow," "archery bow," "fiberglass bow," or the like. Of course, even the phrase "fiberglass bow" may be legitimate for either nautical or archery contexts.

Negative words like "violin," "ship," "ribbon" could be added to further reduce the possibility of irrelevant and unwanted searches and their associated costs.

As an example, not as many people will search for the keyword phrase "phthalate-free bows" as for the keyword "bows," but those who search for the more specific phrase are more likely to be ready to buy from you if your site promotes ribbons and hair bows. Better to have few site visitors who are ready to buy than many site visitors who have little reason to be on one's site (and who will leave in a hurry).

Secondly, to further increase the probability that site visitors will buy, PPC persons try to find and bid on keywords that in themselves have a higher probability of producing sales for their market. Searchers who type in "anniversary gifts" or "arthritis pain relievers" are more likely to be immediately interested in buying products relevant to the keywords than those typing in "Richard Nixon" or "weather."

Third, persons paying for clicks will match their keywords with the words, and hence products and services, offered on their websites. If the keywords paid for are not matched with the corresponding website, the person will pay for the clicks of many who are searching for what is not on the site. And the Search Engine will look bad providing what the searcher does not want.

Thus the person bidding on the phrase "anniversary gifts" may want various kinds of relevant products shown on his or her site: chocolates, flowers, decorative plates, heart-shaped jewelry, frames for photos, or what not. So much the better if the website includes relevant words like "wedding anniversary," "gift ideas," "special," "tenth," "love," and the like. The person bidding on "arthritis pain relievers" may want various brands of pain relievers to appear.