Assignment takes place when the written option is exercised by the options holder (option buyer). The options writer (option seller) is said to be assigned the obligation to deliver the terms of the options contract.
If a put option is assigned, the options writer will have to buy the obligated quantity of the underlying securty at the strike price.
Once an option is sold, there exist a possibility for the option writer to be assigned to fulfil his or her obligation to buy or sell shares of the underlying stock on any business day. One can never tell when an assignment will take place. To ensure a fair distribution of assignments, the Options Clearing Corporation uses a random procedure to assign exercise notices to the accounts maintained with OCC by each Clearing Member. In turn, the assigned firm must use an exchange approved way to allocate those notices to individual accounts which have the short positions on those options.
Options are usually exercised when they get closer to expiration. The reason is that it does not make much sense to exercise an option when there is still time value left. Its more profitable to sell the option instead.
Over the years, only about 17% of options have been exercised. However, it does not mean that only 17% of your short options will be exercised. Many of those options that were not exercised were probably out-of-the-money to begin with and had expired worthless. In any case, at any point in time, the deeper into-the-money the short options, the more likely they will be exercised.
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