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发表于 2013-8-30 12:26 AM
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THETA
Time decay, or theta, is enemy number one for the option buyer. On the other hand, it’s usually the option seller’s best friend. Theta is the amount the price of calls and puts will decrease (at least in theory) for a one-day change in the time to expiration.
Figure 2: Time decay of an at-the-money call option
Time decay trend for an at-the-money call option
This graph shows how an at-the-money option’s value will decay over the last three months until expiration. Notice how time value melts away at an accelerated rate as expiration approaches.
This graph shows how an at-the-money option’s value will decay over the last three months until expiration. Notice how time value melts away at an accelerated rate as expiration approaches.
In the options market, the passage of time is similar to the effect of the hot summer sun on a block of ice. Each moment that passes causes some of the option’s time value to “melt away.” Furthermore, not only does the time value melt away, it does so at an accelerated rate as expiration approaches.
Check out figure 2. As you can see, an at-the-money 90-day option with a premium of $1.70 will lose $.30 of its value in one month. A 60-day option, on the other hand, might lose $.40 of its value over the course of the following month. And the 30-day option will lose the entire remaining $1 of time value by expiration.
At-the-money options will experience more significant dollar losses over time than in- or out-of-the-money options with the same underlying stock and expiration date. That’s because at-the-money options have the most time value built into the premium. And the bigger the chunk of time value built into the price, the more there is to lose.
Keep in mind that for out-of-the-money options, theta will be lower than it is for at-the-money options. That’s because the dollar amount of time value is smaller. However, the loss may be greater percentage-wise for out-of-the-money options because of the smaller time value.
When reading the plays, watch for the net effects of theta in the section called “As time goes by.”
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