One of the simplest ways of estimating Peru's aboriginal population, and one that has been frequently used by scholars, is the depopulation ratio method. Use of depopulation ratios is straightforward. It does not presume an extensive knowledge of statistics.The argument is clearly understandable, and the manipulation of numbers gives an impression of scientific accuracy. The procedure is logical: Known population figures for precontact groups and later census figures for the same population unit are used to derive a ratio. The ratio is then applied to estimate the total regional population from a known census total. In this chapter we shall examine the sources, techniques, and conclusions of four recent investigators who have used depopulation ratios to estimate the Andean native population: John H. Rowe, Henry F. Dobyns, C. T. Smith, and Nathan Wachtel. John H. Rowe was one of the first modern scholars to utilize the depopulation method to calculate the aboriginal population. His estimate is based on five samples: two (Rimac and Chincha) for the coast and three (Yauyos, Huancas, and Soras) for the central highlands (see Table 4). These five cases are employed by Rowe because there are data for both the Inca period (ca. 1525) and the Toledo era (ca.1571).