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Estimation of Employee Stock Option Exercise Rates and Firm Cost: Methodology

Investors have become increasingly concerned about the cost of executive stock options to shareholders. Because executives face hedging constraints, standard option theory does not apply. The valuation problem reduces to accurately characterizing the option payoff. This paper develops a methodology for estimating option exercise and cancellation rates as a function of the stock price path, time to expiration, and firm and option holder characteristics. Our estimation accounts for correlation between exercises by the same executive. Valuation proceeds by using the estimated exercise rate function to describe the option's expected payoff along each stock price path and then computing the present value of the payoff. The estimation of empirical exercise rates also allows us to test the predictions of theoretical models of option exercise behavior. The paper not only illustrates an ideal valuation method for a large dataset, but also shows how to evaluate the usefulness of some of the approximations proposed in the literature.