Thorp decided to approximate it by assuming that the expected price of the stock rose at the riskless rate of interest (the rate paid by a borrower who creditors are certain will not default, for which a usual indicator is the yield of bonds issued in their own currencies by major governments). He had no better estimate, and he "didn't think that enormous errors would necessarily be introduced" by the approximation.

Donald MacKenzie, An Engine, Not a Camera, loc. 557