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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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analysis, it also has its shortcom<strong>in</strong>gs. As we mentioned earlier, duration isquite accurate <strong>in</strong> calculat<strong>in</strong>g changes <strong>in</strong> price for small changes <strong>in</strong> yields. Forlarge changes <strong>in</strong> yields, duration only provides an approximation, and wehave to rely on the concept of convexity for more accurate estimations.CONVEXITYThe true relationship between a change <strong>in</strong> price and a change <strong>in</strong> <strong>in</strong>terestrate for option-free securities is not l<strong>in</strong>ear. The follow<strong>in</strong>g graph demonstratesthis relationship:Error not measured <strong>by</strong> durationPriceActual priceA'Error not measured<strong>by</strong> durationB' Tangent l<strong>in</strong>erepresent<strong>in</strong>g durationYi ¥2 Y Y3 Y 4YieldThe actual price of the security is the curve AA', and the straight l<strong>in</strong>e BB'which is tangent to the AA' curve represents the duration of the security.Therefore, as the graph shows, the straight l<strong>in</strong>e approximation createserrors that grow <strong>with</strong> the magnitude of the <strong>in</strong>terest rate changes. For smallchanges <strong>in</strong> yield, e.g. from Y to Ys or Ys, duration does a good job <strong>in</strong>estimat<strong>in</strong>g the actual price. But for larger changes, e.g. from Y to Yi or Y«,duration becomes less accurate and the errors become larger. Convexityis what we use to correct this problem.Duration and Convexity

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