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Derivatives in Plain Words by Frederic Lau, with a ... - HKU Libraries

Derivatives in Plain Words by Frederic Lau, with a ... - HKU Libraries

Derivatives in Plain Words by Frederic Lau, with a ... - HKU Libraries

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Mathematically, modified duration is the first derivative of bond price whileconvexity is the second derivative. Convexity measures the rate of changeof duration:C XJt = t[ X ( ll + ' ) X PVCF ' + t2 X (t2 + I ) X PVCF2 + ....... + tn X (tn + i ) X PVCFt0nV6XI y( I + yield/k) 2 x (k 2 x PVTCF) * 'The convexity of the security <strong>in</strong> our previous illustration is 20.1886 (follow<strong>in</strong>gthe calculation format illustrated on page 41.)The price change that is due to the curvature is shown <strong>in</strong> the follow<strong>in</strong>gformula:Percentage change <strong>in</strong> price due to convexity =1/2 x convexity x (yield change) 2 x 100 (7)Now we can put th<strong>in</strong>gs together. For a 100 basis po<strong>in</strong>t change <strong>in</strong> yield from8.00 percent to 7.00 percent, the true theoretical price, tak<strong>in</strong>g <strong>in</strong>toconsideration both duration and convexity, is:Percentage change <strong>in</strong> price due to duration = -4.0555 x (-0.01) x 100 = 4.0555%Percentage change <strong>in</strong> price due to convexity = 1/2 x 20.1886 x (-0.0 1) (-0.0 1) x 100= 0.1009%Total percentage change <strong>in</strong> price = 4.0555% + 0.1009%= 4.1564%As you can see, when the <strong>in</strong>terest rate decreases, the convexity actuallyaccelerates the price appreciation. Further, the change <strong>in</strong> price due toconvexity is largely determ<strong>in</strong>ed <strong>by</strong> the change <strong>in</strong> <strong>in</strong>terest rate. If the yieldchange is small, the square of yield change is negligible. If the yield changeis large, this term can be significant and thus affects the outcome of equation(7).Similarly, the true theoretical price change if the <strong>in</strong>terest rate <strong>in</strong>creases from8.00 percent to 9.00 percent can be calculated as follows:Duration and Convexity

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