computer programs scrutinize the pricing of hte related securities. for example, shares of Exxon Mobil and Chevron, historically tend to trade in similar directions, since their fates hinge on oil production and prices. if Exxon's stock starts to rise, but Chevron's doesn't, Medallion might purchase Chevron and sell Exxon short, hoping to benefit as the two stocks move back into alignment.
return is 45% per year from 1988 to now.