1. A method for determining an approximation of a first function (Funct. 1 ) from a second function (Funct. 2 ), where there is a forward solution for calculating the second function from the first function, the first function describing the relationship of a first variable to an independent variable, the second function describing the relationship of a second variable to the independent variable, the second variable being one that is ascertainable, comprising: a) providing the second function (Funct. 2 ); b) applying a shape approach method to produce the approximation of the first function (Funct. 1 ), which characterizes a similarity of two functions by computing an angle between the two functions when they are treated as two vectors in an n dimensional space where n is a number of data points to produce the approximation of the first function (Funct. 1 ) including: comparing a shape of the second function (Funct. 2 ) with a shape of a reconstructed second function (Funct. 2 A) derived from a proposed first function (Funct. 1 a ), the proposed first function describing a proposed relationship of the first variable to the independent variable; c). optionally, repeating step b a finite number of times.