Let us express the process of calculating approximate percentage price change for a given change in yield and a given duration using the following formula:

         Approximate percentage price change = - duration × Δy ×100                      ...Eq. (2)

As an inverse relationship exists between the price change and yield change, duration is preceded by a minus sign.

For example, let us take a 20-year bond with 8.5% coupon rate, currently trading at Rs.115.89 whose duration is 10.22. The approximate percentage price change for a 20 basis point increase in yield (i.e., Δy =  +0.002) is

         Approximate percentage price change = -10.22 × (+0.002) ×100 = -2.044%