The Bachelier model, published by Louis Bachelier in 1900, assumes that the forward rate follows : it moves by normally distributed amounts, independent of its level, and can go negative. Its parameter , in basis points a year, is the normal volatility; the implied normal volatility of an option is the at which the model reproduces its price.
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Example 13.6 (A straddle)
On a flat 4% curve the 1y10y forward annuity is 7.80. At 95 basis points of normal volatility, a straddle on USD 100 million costs of notional, USD 5.91 million. Divided by the annuity it is 75.8 basis points: the ten-year rate must end more than about 76 basis points away from 4% for the straddle held to expiry to pay for itself.
Example 13.2 (A cap and a zero-cost collar)
On a flat 4% curve, a five-year cap at 4.5% on USD 100 million of an annual rate, with the first period already fixed, has four caplets fixing in one to four years. At a normal volatility of 100 basis points a year they are worth USD 182 874, 310 339, 401 400 and 470 709: USD 1.37 million in all, 137 basis points of notional. A floor at 3.5% is worth exactly the same, since the forward sits halfway between the strikes and the normal model is symmetric: the borrower who buys the cap and sells the floor pays nothing and keeps its rate between 3.5% and 4.5%.