One concern we hear frequently is whether it is beneficial or harmful to repeat keywords. In other words, should we vary keywords (dog food, puppy food, and Purina) or repeat keywords (dog food reviews, dog food comparison, and dog food rankings.) The short answer is that the repetition is just fine, as long as the meaning of the phrase as a whole is sufficiently varied. In other words, dog food and dog food online are basically synonymous, and the content that one might expect to find associated with both keywords is the same. However, dog food reviews and dog food comparison indicate somewhat different content and therefore are appropriate to be used in tandem as keywords.
Because someone who is looking for something that specific is probably a much more qualified searcher for your product or service (presuming you're in the blogging space) than someone looking for something really generic. And because long-tail keywords tend to be more specific, it's usually easier to tell what people who search for those keywords are really looking for. Someone searching for the head term "blogging," on the other hand, could be searching it for a whole host of reasons unrelated to your business.