Microsoft has announced The greatest obstacle with personalizing results is the unpredictability of human behavior. "We think one of the challenges with delivering results which are truly individualized is that, to date, personalized search can't see the forest for the trees. In other words everyone is collecting everything and trying to figure out the foibles of human behavior from a mass of digital bits. To an extent, we've all been looking at the wrong inputs which in turn haven't given us the output we want."