assumes that they remain constant, computing a constant multiplier, the resulting predictions are valid only if these ratios do not in fact change. Sometimes this holds, and sometimes it does not; for example, increases in central bank money may result in increases in commercial bank money – and will, if these ratios (and thus multiplier) stay constant – or may result in increases in excess reserves but little or no change in commercial bank money, in which case the reserve–deposit ratio will grow and the multiplier will fall. Mechanism There are two suggested mechanisms for how money creation occurs in a fractional-reserve banking system: either reserves are first injected by the central bank, and then lent on by the commercial banks, or loans are first extended by commercial banks, and then backed by reserves borrowed from the central bank. The "reserves first" model is that taught in mainstream economics textbooks, while the "loans first" model is advanced by endogenous money theorists. Reserves first model In the "reserves first" model of money creation, a given reserve is lent out by a bank, then deposited at a bank (possibly different), which is then lent out again, the process repeating and the ultimate result being a geometric series. Formula The money multiplier, , is the inverse of the reserve requirement, : General formula To correct for currency drain (a lessening of the impact of monetary policy due to peoples' desire to hold some currency in the form of cash) and for banks' desire to hold reserves in excess of the required amount, the formula: can be used, where "Currency Drain Ratio" is the ratio of cash to deposits, i.e. C/D, and the Desired Reserve Ratio is the sum of the Required Reserve Ratio and the Excess Reserve Ratio. The desired reserve ratio is the amount of its assets that a bank chooses to hold as excess and required reserves; it is a decreasing function of the amount by which the market rate for loans to the non-bank public from banks exceeds the interest rate on excess reserves and of the amount by which the federal funds rate exceeds the interest rate on excess reserves. Since the money multiplier in turn depends negatively on the desired reserve ratio, the money multiplier depends positively on these two opportunity costs. Moreover, the public’s choice of the currency drain ratio depends negatively on market rates of return