 Another generalizing of Nelson-Siegel is the family of Exponential Polynomial Model ("EPM(n)") where the number of linear coefficients is free.Once a curve has been fitted, the user can then define various measures of shift, twist and butterfly, and calculate their values from the calculated parameters.For instance, the amount of shift in a curve modeled by a polynomial function can be modeled as the difference between the polynomial formula_30 parameters at successive dates.