You should generally choose the longer form of a keyword to take advantage of a concept called “word stemming.” When a search engine uses word stemming, it searches for word roots will also include the variations of that word. For instance, searches for “consult” would also return documents that have the words “consulting” and “consultants” unless the user chooses to do an exact search.
The ranking criteria is different among search engines, but most grade the placement of keywords on your Web site, the site’s title and description based on these factors:
1. Prominence of the keyword
2. Frequency of the keyword
3. Site popularity
4. Weight of the keywords
5. Proximity of keywords
6. Keyword placement
Keyword Prominence: The first place the keyword appears in the Web site’s title or description. Did the title of the site start with a particular keyword or was that keyword the fourth or fifth word of the site’s title?
Keyword Frequency: The number of times a keyword appears in a site’s title or description. If you repeat a word too many times, you may be penalized for keyword stuffing. Generally, you should repeat your keyword in the document as many times as possible without allowing it to distract from your copy (text). It should also be stated from three to seven times in your META tags.
Site Popularity: The number of other Web sites linked to yours. This ranking measurement gives credit to those sites that were linked to because they were deemed important. More weight is given to sites relevant to your own and those with higher rankings.
Keyword Weight: The number of keywords appearing on a Web page compared to the total number of words appearing on that page. Some search engines consider this when determining the rank of your Web site for a particular keyword search.
Keyword Proximity: The placement of keywords in relation to each other or other words with a similar meaning as the queried keyword. For search engines that grade a keyword match by keyword proximity, the connected phrase “home loans” will outrank a citation that mentions “home mortgage loans” if you are searching only for the phrase “home loans.”
Keyword Placement: Where on your page your keywords are located. For example, in most engines, placing the keywords in the title tag of the page or in the heading tag will give it more relevancy. On some engines, placing keywords in the link text, the part that is underlined on the screen in a browser, can add more relevancy to those words.