When Exercising an Option Makes Sense
12/08/2009 12:01 am EST
When you buy an option, you get the right to exercise that option, so I'd like to discuss when it makes sense to take advantage of that right. Of course, I'm talking about American-style options here, because European options can only be exercised at expiration.
When a long option position is put on, there are several things that can be done to close the position:
1) You can sell the option
2) You can exercise the option
3) You can hold the option to expiration, at which time it will either be worthless or equal to parity (stock – strike for a call, and strike – stock for a put.)
I'm sure you know that an options' value is made up of both intrinsic value and time value. When an option is exercised, any remaining time value is lost.
So our first criteria for when to early exercise is for the option to be deep in the money and have a delta very close to 100. A 98 or 99 delta option may also be a candidate, but the closer to 100, the better.
When to Exercise
In order for it to make sense to early exercise an option, there must be some positive cash flow that results from the exercise. We'll look at calls and puts separately to determine when the early exercise is the right way to go.
First, we'll need to remember two of the six basic synthetic equations:
Long Call = Long Stock + Long Put
Long Put = Short Stock + Long Call
Let's assume that we had no position on. Well, no position is the same as having a long call, and also having the exact same short call on at the same time. So to put this into the form of an equation, we can say:
Long Call + Short Call = 0
Now remember, we want to know when we would get a positive cash flow by exercising the long call, i.e., under what circumstances will exercising the call give us a value greater than 0.
Or, when is the Exercised Long Call + Short Call > 0
What happens when the long call is exercised?
We lose the call and we get long stock and any dividends (D), but we have to finance the stock by paying interest on the strike price from now through expiration. We'll call this "I."
So substituting for the exercised long call we get:
Long Stock + D – I + Short Call > 0
Recognizing that the long stock and short call is synthetically a short put, we get:
Short Put + D – I > 0, and then, transposing the short put to the other side of the inequality (where it then becomes a long put), we finally get the result…phew!
D – I > Long Put
MORE: Early Exercising Dividend vs. Non-Dividend Paying Stocks|pagebreak|
Dividends and Exercise
So what does it mean?
First, in the case of a stock that does not pay dividends, i.e., D = 0, it never pays to exercise a call early. If the stock does pay a dividend, we would want to wait as long as possible before exercising (to minimize I), but still capture the dividend.
This, of course, happens on the day before the stock goes ex-dividend. Don't worry if you didn't completely follow how we got there, however, the results are very important and are boldly restated:
On a non-dividend paying stock, it never pays to early exercise a call option.
On a dividend-paying stock, the only time it may pay to exercise a call option is the day before the stock goes ex-dividend, and only if the dividend minus the cost of carry is less than the corresponding put.
Basically, you can think of it as exercising the call, under the right conditions, to get the dividend.
OK, what about puts? When do we exercise them?
The demonstration is very similar, so I'll go through it without all the explanation
Long Put + Short Put = 0
Exercised Long Put + Short Put > 0
Short Stock –D + I + Short Put > 0
Short Call – D + I > 0
I – D > Long Call
The results here are a little different, and you have to be a little more careful. If the stock is non-dividend paying, then the only time you would exercise the put is when the cost of carry is greater than the corresponding call.
When would this be likely to occur?
The put would have to be deep in the money, and some combination of high interest rates and low volatility.
When the stock does pay a dividend, then the time to check would generally be after the dividend is paid, i.e., after the ex-dividend date when it's now equal to 0, although if the dividend is small enough, there are situations where it might make sense to exercise the put even before the expiration date.
A Final Point
Here's a final point on early exercise: I've had students tell me that they don't want to short in-the-money options for fear of being assigned. Let's put that one to bed. If you're short a deep-in-the-money option and you get assigned, you're basically in the same position. The option you were short had a delta close to 100 for puts and -100 for calls.
So when you find out you've been assigned and that it creates a cash-flow issue, you call your broker and either sell the long stock and sell another, less deep-in-the-money put, or buy back the short stock and sell a less deep-in-the-money call.
Truth is, if you are short deep-in-the-money options that meet the criteria for being exercised and they're not assigned, then that's actually a benefit for you. In fact, this concept is what "dividend plays" are all about, but that's for another article!
By Stan Freifeld of Online Trading Academy