During severe deflation, targeting an interest rate (the usual method of determining how much currency to create) may be ineffective, because even lowering the short-term interest rate to zero may result in a real interest rate which is too high to attract credit-worthy borrowers. In the 21st-century negative interest rate has been tried, but it can't be too negative, since people might withdraw cash from bank accounts if they have negative interest rate. Thus the central bank must directly set a target for the quantity of money (called "quantitative easing") and may use extraordinary methods to increase the supply of money, e.g. purchasing financial assets of a type not usually used by the central bank as reserves (such as mortgage-backed securities). Before he was Chairman of the United States Federal Reserve, Ben Bernanke claimed in 2002, "...sufficient injections of money will ultimately always reverse a deflation", although Japan's deflationary spiral was not broken by the amount of quantitative easing provided by the Bank of Japan.