Google provides tools that allow you to select those that best fit the activity on your site. But, we need a comprehensive statistical study to see which keywords are most successful niche. By example, suppose you sell clothes for men. Your keywords may be many: jacket, black leather jacket, pants, denim, cotton, pullover, shirt, etc. But after making his study of the frequency of occurrence of keywords, it happens that sweater is rarely sought, compared with, for example, sweater. Then, rate a Google Adwords campaign with the word sweater would be a mistake. On the other hand, it happens that there are too many advertisers using the word sweater, which makes this keyword have much competition. Then, it would be a mistake to focus your Google Adwords campaign in this particular word: as listed out in the natural results under many other sites is not, nor is that your Google Adwords receives notice of an impression (ie is shown), down from nine other notices.