Popularity
By far the easiest of the three to judge is popularity, since it is not subjective. Software like WordTracker gives popularity figures of search phrases based upon actual search engine activity (it also gives additional keyphrase suggestions and variations). Such software allows you to assign a concrete popularity number to each phrase to use when comparing them. Obviously, the higher the number, the more traffic that can be expected (assuming you are able to obtain good search engine positions). However, this number alone is not good enough reason to pursue any particular keyphrase, although too often keyphrase analysis stops here.

Specificity
This is more abstract than the sheer popularity number, but equally important. For example, let's assume that you were able to obtain great rankings for the keyphrase "insurance companies" (a daunting prospect). Let's also assume that you only deal with auto insurance. Although "insurance companies" might have a much higher popularity figure than "auto insurance companies", the first keyphrase would also be comprised of people looking for life insurance, health insurance, and home insurance. It is very likely that someone searching for a particular type of insurance will refine their search after seeing the disparate results returned from thephrase "insurance companies". In the second, longer keyphrase, you can be reasonably sure that a much higher percentage of visitors will be looking for what you offer- and the addition of the word "auto" will make it much easier to attain higher rankings, since the longer term will be less competitive.

Motivation of User
This factor, even more abstract than specificity, calls for an attempt to understand the motivation of a search engine user by simply analyzing his or her search phrase. Assume, for example, that you were a real estate agent in Atlanta. Two of the keyphrases you are evaluating are "Atlanta real estate listings" and "Atlanta real estate agents". Both phrases have very similar popularity numbers. They are also each fairly specific, and your services are very relevant to each. So which phrase is better? If you look into the likely motivation of the user, you will probably conclude that the second is superior. While both phrases target people looking for real estate in Atlanta, you can infer from the second phrase that the searcher has moved beyond the point where they are browsing local homes or checking out prices in their neighborhood- they are looking for an agent, which implies that they are ready to act. Often, subtle distinctions between terms can make a large difference on the quality of the traffic they attract.

3. Evaluating Keyphrase Performance:
Until recently, judging the performance of individual keyphrases was a dicey proposition. Although it is possible to tell from your log traffic analysis how many visitors are getting to your site from each keyphrase (valuable information, but unfortunately not enough to do much with), it was very hard to decipher which phrases were bringing you the most quality traffic. Recently, however, some sophisticated but affordable tools have been developed that allow you to judge the performance of each individual keyphrase based upon visitor behavior. This new software makes it possible to periodically analyze which keyphrases are bringing your site the most valuable visitors- those who buy your products, fill out your contact form, download your demo, etc. This type of data, rather than the sheer number of visitors from each search phrase alone, is invaluable when you are refining your search engine marketing campaigns, since you can discard and replace non-performing keyphrases and put increased effort toward the phrases that are delivering visitors that become customers. This kind of ongoing analysis is the final piece of the keyphrase puzzle, and allows you to continually target the most important phrases for your industry, even if they change over time. 

Conclusion:
Keyphrase compilation, evaluation, and performance are all vitally important to any search engine marketing campaign.  While high rankings in search engines are an admirable goal, high rankings for poor keyphrases will consistently deliver poor results. Integration of this keyphrase process into your overall search engine marketing strategy can dramatically improve your website performance (and thus your bottom line).
 

Scott Buresh is co-founder and principal of Medium Blue Internet Marketing (www.mediumblue.com). For monthly tips on how to get the most out of your internet presence, sign up for the Medium Blue Internet Marketing Newsletter at www.mediumblue.com/newsletters.