Let's take the 3 big SE's and have a look at their Search Suggestions:
Bing, Google and Yahoo!.
When you begin typing in the search box on the homepage of each search engine, a suggestion list appears. The more you type, the more the suggestions change (or get added to).
Are these search suggestions based on the most popular searches performed by others, or are these search suggestions based on relevance (what it may "think" you're going to type next)?
So basically, it is popularity or relevance? Or is it a smart combination of both? And if possible, can you please back your answers up with any supporting documentation?