If you think too much about it, you might conclude that certain is an absolute term like unanimous or paramount and cannot be modified. Something is either certain or it is not, you might say. There can be no in-between. But before you say much more, you may find yourself using certain in combination with adverbs such as fairly, absolutely, and completely, which would seem to imply that levels of certainty exist. In an earlier survey, a majority of the Usage Panel accepted the construction Nothing could be more certain, so you can be confident that modifying certain is a pretty safe bet.