Abstract
A new prepayment model is developed, which improves the modeling of the borrowers decision process by incorporating an occupation-time derivative in the valuation framework of a fixed-rate mortgage. This option-theoretic mortgage valuation model is based on stochastic house-price and interest-rate models, and requires a particularly subtle technique to incorporate a new type of occupation-time derivative, where the barrier (which activates the derivative) is in the value process and not the underlying process (as it is in standard occupation-time derivatives). This new model simulates a delay in prepayment by the borrower (beyond the time simple ruthless prepayment dictates), thus increasing the value of the mortgage to the lender, compared to the value gained using more basic models. This allows for a more advanced borrower decision process, where a rational exercise structure is retained in a modified form. Empirical evidence supports this theory, which should be beneficial for accurate mortgage-backed security pricing. The results in this paper explore thoroughly the effect on the mortgage value of a delay in prepayment by the borrower on the embedded options held and on the insurance component.
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Notes
Haber et al. (1999) define the dynamics of barrier time \(\bar{t}\) so that it increases at the same rate as the real time t, therefore \(d\bar{t}=dt\), if the underlying S is beyond the barrier. The barrier time \(\bar{t}\) is reset to zero if S hits the barrier \(S=\bar{S}\), and does not change if \(S<\bar{S}\).
Convergence of the boundary location procedure is guaranteed due to the way the discrete approximation is found. As the check for V ≥ FV is performed for increasing values of interest rate from r = 0 and it is known that prepayment region expands as we move away from the maturity of the mortgage (the same direction as the valuation procedure), the discrete approximation of the free boundary is always captured.
When the mortgage value referred to is not the true value inverted commas are used to mark the distinction.
Results were taken for longer maturities but were not qualitatively different.
The results were obtained using a 2412 MHz AMD Athlon computer.
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Acknowledgements
The research of the first and second authors was supported by the EPSRC. The authors acknowledge a number of useful and insightful comments raised by the reviewers of a previous version of this paper.
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Sharp, N.J., Johnson, P.V., Newton, D.P. et al. A New Prepayment Model (with Default): An Occupation-time Derivative Approach. J Real Estate Finan Econ 39, 118–145 (2009). https://doi.org/10.1007/s11146-008-9105-7
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DOI: https://doi.org/10.1007/s11146-008-9105-7