There are two kinds of indexes for the environmental assessment. Those are total amount and ratio. Both of them have strong point and weak point.
For example, it is the amount of input and output of chemical substances in a factory.
Density of chemical substances and the basic unit of CO2 are examples.
Correlation is considered when we use this index.
Eco efficient has some definitions. For example, "Output / Input".
WBCSD (World Business Council for Sustainable Development) suggests the criterion for Eco Efficiency.
As eco efficiency, there is a definition,
"Value / Environmental-Impact".
The dimension of this index is not zero because dimensions of the numerator and the denominator are different.
Factor is defined
"Eco efficiency of the target / Eco efficient of the standard".
Factor is zero dimension.