To ensure fairness in the distribution of equity and index option assignments, OCC utilizes a random procedure to assign exercise notices to the accounts maintained with OCC by each Clearing Member. The assigned firm must then use an exchange approved method (usually a random process or the "first-in, first-out" method) to allocate those notices to accounts which are short the options.Quote from BellWeather:
The chance of being exercised on the short portion(s) of the fly is.......nonexistent??? Last in...First out?
http://www.optionseducation.org/tools/faq/options_assignment.html