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We now describe a cash management system designed to deal with cash inflows and outflows
that fluctuate randomly from day to day. With this model, we again concentrate on the cash balance. But in contrast to the situation with the BAT model, we assume that this
balance fluctuates up and down randomly and that the average change is zero.
The Basic Idea Figure 20A.3 shows how the system works. It operates in terms of an upper limit and a lower limit (L) to the amount of cash (U*), as well as a target cash
balance (C*). The fi rm allows its cash balance to wander around between the lower and upper limits. As long as the cash balance is somewhere between U* and L, nothing happens.
When the cash balance reaches the upper limit (U*), as it does at point X, the firm moves U* C* dollars out of the account and into marketable securities. This action moves the cash balance down to C*. In the same way, if the cash balance falls to the lower limit (L),as it does at point Y, the firm will sell C* L worth of securities and deposit the cash in the account. This action takes the cash balance up to C*.
Using the Model To get started, management sets the lower limit (L). This limit essentially defines a safety stock; so where it is set depends on how much risk of a cash shortfall the firm is willing to tolerate. Alternatively, the minimum might just equal a required compensating balance.
As with the BAT model, the optimal cash balance depends on trading costs and opportunity
costs. Once again, the cost per transaction of buying and selling marketable securities,
F, is assumed to be fixed. Also, the opportunity cost of holding cash is R, the interest rate per period on marketable securities.
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