Several empires in world history have been contenders for the largest of all time, depending on definition and mode of measurement. Possible ways of measuring size include area, population, economy, and power. Of these, area is the most commonly used because it has a fairly precise definition and can be feasibly measured with some degree of accuracy.Estonian political scientist Rein Taagepera, who published a series of academic articles about the territorial extents of historical empires between 1978 and 1997,defined an empire as "any relatively large sovereign political entity whose components are not sovereign" and its size as the area over which the empire has some undisputed military and taxation prerogatives;[6] these are the criteria from which these lists are formed. The list is not exhaustive owing to a lack of available data for several empires; for this reason and because of the inherent uncertainty in the area estimates, no rankings are given.