Friedman's k-percent rule is a monetary policy rule that the money supply should be increased by the central bank by a constant percentage rate every year, irrespective of business cycles. In "A Monetary History of the United States, 1867–1960", monetarist economists Milton Friedman and Anna Schwartz attributed inflation to excess money supply generated by a central bank. It attributed deflationary spirals to the reverse effect of a failure of a central bank to support the money supply during a liquidity crunch. Friedman proposed a fixed monetary rule, called Friedman's k-percent rule, where the money supply would be calculated by known macroeconomic and financial factors, targeting a specific level or range of inflation.