instrument. Variable energy form, but constant energy flow The situation becomes more complex when the form of energy does not remain constant. In this context Ohta formulated the question of energy quality in terms of the conversion of energy of one form into another, that is the transformation of energy. Here, energy quality is defined by the relative ease with which the energy transforms, from form to form. If energy A is relatively easier to convert to energy B but energy B is relatively harder to convert to energy A, then the quality of energy A is defined as being higher than that of B. The ranking of energy quality is also defined in a similar way. (Ohta 1994, p. 90). Nomenclature: Prior to Ohta's definition above, A. W. Culp produced an energy conversion table describing the different conversions from one energy to another. Culp's treatment made use of a subscript to indicate which energy form is being talked about. Therefore, instead of writing "energy A", like Ohta above, Culp referred to "Je", to specify electrical form of energy, where "J" refers to "energy", and the "e" subscript refers to electrical form of energy. Culp's notation anticipated Scienceman's (1997) later maxim that all energy should be specified as form energy with the appropriate subscript. Energy quality in biophysical economics (indirect energy transformations) The notion of energy quality was also recognised in the economic sciences. In the context of biophysical economics energy quality was measured by the amount of economic output generated per unit of energy input (C.J. Cleveland et al. 2000). The estimation of energy quality in an economic context is also associated with embodied energy methodologies. Another example of the economic relevance of the energy quality concept is given by Brian Fleay. Fleay says that the "Energy Profit Ratio (EPR) is one measure of energy quality and a pivotal index for assessing the economic performance of fuels. Both the direct and indirect energy inputs embodied in goods and services must be included in the denominator." (2006; p. 10) Fley calculates the EPR as the energy output/energy input. Ranking energy quality Energy abundance and relative transformation ease as measure of hierarchical rank and/or hierarchical position 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.