Ohta sought to order energy form conversions according to their quality and introduced a hierarchical scale for ranking energy quality based on the relative ease of energy conversion (see table to right after Ohta, p. 90). It is evident that Ohta did not analyse all forms of energy. For example, water is left out of his evaluation. It is important to note that the ranking of energy quality is not determined solely with reference to the efficiency of the energy conversion. This is to say that the evaluation of "relative ease" of an energy conversion is only partly dependent on transformation efficiency. As Ohta wrote, "the turbine generator and the electric motor have nearly the same efficiency, therefore we cannot say which has the higher quality" (1994, p. 90). Ohta therefore also included, 'abundance in nature' as another criterion for the determination energy quality rank. For example, Ohta said that, "the only electrical energy which exists in natural circumstances is lightning, while many mechanical energies exist." (Ibid.). (See also table 1. in Wall's article for another example ranking of energy quality).