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I'm still really new to options but I'm trying to understand this concept of assignment.

Let’s say I’m short a call of Microsoft at a strike of 80 set to expire in a quarter. Microsoft surges for one day, going up to 86. Someone long a call for the same option exercises early. It’s randomly assigned who has to physically settle with this buyer. And so as an option seller, you can’t guarantee you will hold the option until expiration, and you can be randomly assigned to someone who wants to exercise early? So you're taking the risk that you will be randomly assigned someone long the same option who wants to exercise early? How is this fair, as theoretically different traders short a call for the same strike may receive different profits based on if one is exercised early?

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Actually, the process is not completely random. Rather it follows the OCC's assignment procedures: https://www.theocc.com/components/docs/legal/rules_and_bylaws/standard-assignment-procedures.pdf

The broker who is assigned in turn allocates their assignments among their customer accounts according to their own procedures (which could be random allocation, first-in first-out, etc).

You can also look at how much assignment activity is going on by the change in open interest from one day to the next compared with transaction volume. Many participants will also exercise options into order to receive dividends before the stock goes ex-dividend, but as @JoeTaxpayer has pointed out, most people will hold options to expiration.

The OCC have a policy where at expiration, if an option is in-the-money by $0.01 or more, then they will automatically exercise the option, unless they receive "contrary advice" from the holder.

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You've described the process fairly well.

It's tough to answer a question that ultimately is 'how is this fair?' It's fair in that it's part of the known risk. And for the fact that it applies to all, pretty equally.

In general, this is not very common. (No, I don't have percents handy, I'm just suggesting from decades of trading it's probably occurring less than 10% of the time). Why? Because there's usually more value to the buyer in simply selling the option and using the proceeds to buy the stock. The option will have 2 components, its intrinsic value ("in the money") and the time premium. It takes the odd combination of low-to-no time premium, but desire of the buyer to own the stock that makes the exercise desirable.

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    The more common case for a call option being exercised early is right before the ex-date of a dividend on the underlying stock. If the time premium is less than the amount of the dividend, then it will be exercised. – DanTilkin Nov 1 '17 at 19:14
  • Exactly! I should have spelled that out. – JoeTaxpayer Nov 1 '17 at 19:16
  • According to OCC, in 2017 only 7% of option contracts were exercised. It's poorly phrased to say that calls will be exercised early for the dividend because the call's time premium is less than the dividend (that's only true for the put). With the stock at $100, would anyone exercise deep OTM $105 or $110 calls for that reason? Even if ITM, it makes no sense because the arb isn't profitable. AFAIC, this is caused by call sellers selling in advance of ex-div and driving the bid below parity, leading to Discount Arbitrage. – Bob Baerker Oct 21 '18 at 16:28
  • Some time ago, I got exercised early. And the timing implied they were capturing a dividend. Otherwise they just could have sold their option and kept some time premium. – JoeTaxpayer Oct 21 '18 at 17:52

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