# Is this simple model used to calculate the interest rate duration and credit duration of a floating rate note? Other models?

I found this model for floating rate bonds in a book I am reading and I am wondering if it is used anywhere in practice?

$$MV=\frac{\frac{(Index+QM)\cdot FV}{PER}}{\left(1+\frac{Index+DM}{PER}\right)^1}+\frac{\frac{(Index+QM)\cdot FV}{PER}}{\left(1+\frac{Index+DM}{PER}\right)^2}+\cdots+\frac{\frac{(Index+QM)\cdot FV}{PER}+FV}{\left(1+\frac{Index+DM}{PER}\right)^N},$$

where MV= market value, Index = reference rate, QM = Quoted Margin, FV= face value, PER= periodicity, N=number of periods to maturity. The important point here is that we put in todays value of Index everywhere, even though it will change.

In this model I assume you have MV, Index is the current rate, and you have QM and FV, then you solve for DM. The calculated interest rate duration is:

$$\frac{MV(Index-\Delta Index)-MV(Index+\Delta Index)}{2\Delta Index \cdot MV},$$ and to calculate the credit duration you have

$$\frac{MV(DM-\Delta DM)-MV(DM+\Delta DM)}{2\Delta DM \cdot MV}.$$

Is this model and these equations used in the real world to calculate interest rate duraiton and credit duration? If not, could you please tell me which model is used and how these durations are calculated?