Which of the following is NOT the assumption of Baumol's model of cash management?

William J. Baumol's inventory-theoretic model of cash management (1952) explicitly assumes certainty—namely, that the firm is able to forecast its cash needs with complete certainty, cash disbursements occur uniformly over time, the opportunity (holding) cost of cash is known and constant, and each conversion of marketable securities into cash incurs a fixed transaction cost. Therefore, inability to forecast cash needs with certainty is NOT an assumption of Baumol's model.