Why you need the links you're not actually targeting...
When the former US Secretary of Defense Donald Rumsfeld famously articulated his beliefs on "known knowns", "known unknowns" and "unknown unknowns" his speech was met equally with instant confusion and derision by many spectators. Yet, somehow, the sentiments packed in his oration have, with time, not only gained appreciation but also become applicable to much of the wider world and, in the context I'm about to appropriate his verbal dexterity for, somewhat surprisingly in the world of SEO.
The first of his maxims is the most self explanatory of the three. "Known knowns" are, quite simply, facts which we are aware of. In SEO a "known known" would constitute the fact that a keyword heavy anchor text link represents higher value in getting a website rated higher in search engines than, for example, a URL string. It is a self evident truth that some links are more valuable than others. It is also a "known known" that links from the BBC's website are worth a lot more, at the very minimum in SEO terms, than those accrued from article databases. Similarly white hat links are worth much more than those accrued through black hat techniques – if you're not caught at first it is more than likely you will be and, as Benjamin Franklin once said, "A good conscience is a continual Christmas."
'Known Unknowns' in SEO
In the SEO world once we have gotten past "known knowns" things get a bit more complicated. One of the "known unknowns" is the exact specifics of how Google creates its search rankings. We know that its algorithms appreciate trusted sources and this can be measured, using Page Ranks and the like, in a quite specific quantitative fashion. What we do not know, or more specifically what we know we do not know about SEO, is that although we are aware that Google's spiders search out for "organic" results we are not sure exactly, in quantitative measures, what this constitutes (for a good look at SEOs speculating these factors check here). There are numerous examples of a company finding themselves penalised for their search results looking highly manipulated; an example would be a web site where every single one of their incoming links constitutes Do Follow links sharing the exact same phraseology of keyword in their anchor text. However, the "known unknown" in this instance in what the tipping point is for this to occur; the specific ratio of exact keyword terms, although it is believed to be a large percentage, is not certain.
With this in mind there are several factors that need to be taken into consideration. What this suggests is that not every link you pursue should be the "perfect" link – do not always go for high PR Do Follow links with perfectly phrased keywords in the anchor text. In fact, according to SEOMoz's latest poll, NoFollow links can actually help with search engine rankings when used correctly. Previously considered a "known known" that No Follow links would not affect search engine rankings it is now a "known unknown" as to what their exact effect may be. The SEOMoz polling found a direct positive correlation between No Follow links and better search ranking results so, due to this, it is worth asking exactly what actions should be taken now with No Follow links. It can be speculated now that at the very minimum they have a neutral impact but suggestions find they may actually positively affect these listings so, at the very minimum, a free No Follow link should not be turned down. This suggests that a sprinkling of imperfect links can actually guarantee better results than pursuing the top links all the time.
Pulling back from political doublespeak for a moment, what does any of this mean in terms of SEO? In short the latest findings hint that, although high PR sites and anchor text keywords should remain priority, a few lesser links – one's that might potentially seem worthless – should be added here or there. To end on a more simple metaphor imagine link building like a plate of chips – whilst the potatoes represent the high PR links, and should be the focus of the meal, try adding a few more ingredients. It is true that a plate consisting of vinegar or salt or ketchup alone – the No Follow links, the generic anchor text - would constitute a poor meal, but, if added sparingly they will certainly add much needed flavour to the main course.