Breaking down each bond into coupons and principal in order to discount each flow at the correct rate like the textbooks say just to be theoretically correct and account for stuff like higher coupon bonds having lower yields under steep curves seems rather futile when in reality high coupon BTPs actually had higher yields and sometimes results of the fitting experiment aren’t even repeatable (due to different starting point). Working with YTMs directly is a lot more convenient. I tried to build the curve for various points between 2009 and 2014, and for last Friday. The results are entirely different from Bank of Italy and Bloomberg curves. The good thing is that there no sharp jumps in coefficients and no stuck points, at least if I stick to the same starting point in each experiment.
For a few semi-random time points picked the curves were quite smooth and residuals were on average 3-8 basis points except 2011H2. Early December 2011 was particularly ugly with bumps in the curve, 18 basis points average residual and entirely different very long-end model yield based on the fitting process starting point.
For now the BTP futures CTD yield is 8 bps above the model value.
For a few semi-random time points picked the curves were quite smooth and residuals were on average 3-8 basis points except 2011H2. Early December 2011 was particularly ugly with bumps in the curve, 18 basis points average residual and entirely different very long-end model yield based on the fitting process starting point.
For now the BTP futures CTD yield is 8 bps above the model value.