be 这个option让stata以组为分析单位，此时的个案数为分组数。此时，用到的观测其实是y的各组均值和x的各组均值。以下两个命令等价。 1、xtreg y x,be i(groupvar) 2a、collapse (mean) y x,by(groupvar) 2b、reg y x fe这个option计算固定效应，以下两个命令等价： 3、xtreg y x,fe i(groupvar) 4、reg y x i.groupvar 对于re vs. fe，一般，基于panel data的回归分析需要考虑用fixed effect还是random effect，有时文章同时将两者的结果列出。通过hausman test，作者可以选取更适合的那种具体在文中解释。
Example 8 - Black-Scholes The value of a European “call option,” c (s,t ) , with exercise price e and expiration date T , satisfies the “asset-or-nothing payoff ” c (s,T ) = s, s ≥ e;= 0, s e . Prior to expiration c (s,t ) is estimated by the Black-Scholes differential equation ( ) ( ) 2 2 2 2 2 0 2 2 t ss s t s s s c s c rsc rc c s c r sc rc σ σ + + − ≡ + + −σ − = . The parameters in the model are the risk-free interest rate, r , and the stock volatility,σ . The boundary conditions are c (0,t ) = 0 and ( , ) 1, s c s t ≈ s →∞ . This development is described in Wilmott, et al. (1995), pages 41-57. There are explicit solutions for this equation based on the Normal Curve of Probability. The normal curve, and the solution itself, can be efficiently computed with the IMSL function ANORDF, IMSL (1994), page 186. With numerical integrationthe equation itself or the payoff can be readily changed to include other formulas, c (s,T ) , and corresponding boundary conditions. We use e = 100,r = 0.08,T − t = 0.25,σ2 = 0.04,sL = 0, and sR = 150. P1069——Fortran—Math
In-the-money: A call option is in-the-money when its strike price is below the current trading price of the underlying asset. A put option is in-the-money when its strike price is above the current trading price of the underlying asset. In-the-money options are generally more expensive as their premiums consist of significant intrinsic value on the top of their time value. Out of money option: An option without any intrinsic value is an out-of-the-money (OTM) option. A call option is out-of-the-money when the strike price is above the current trading price of the underlying security. Intrinsic value: option value if it expired immediately. Intrinsic call value=max(S-X,0). Intrinsic value 0, then the option is in the money. Speculative Value: Speculative Value=Option Price - Intrinsic Value
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