The simplest statement to make about the lags in monetary policy is they are long and variable. This simple statement is also the key insight to understanding the actual implementation of monetary policy. Hence, how many months or years in advance must a central bank forecast to achieve its monetary goals? In 1994, the Economist said:
But [central banks] cannot afford to wait until inflation is actually rising before they act. Monetary policy does not change the speed of the economy instantly: it can take 18 months or more for a rise in interest rates to have its full impact on inflation. The target of policy ought therefore to be future not current inflation, in order to prevent a surge in 1996. The earlier interest rates are raised, the better the chances of engineering a smooth slowdown to a sustainable rate of growth before slack in the economy is exhausted.
Economists differ about the length of those lags. Uncertainty about the average length of those lags and the variability of those lags makes discretion most difficult. Activist policy can improve welfare only if the information about economic structure and economists’ ability to forecast is sufficiently accurate.

Friedman is the most famous and persuasive critic of Keynesianism on the grounds of lags. He has two main arguments: first, that there are “long and variable lags” between the identification of a problem and the effects of the designed remedy; second, that activist policy often itself becomes a source of instability since policy itself becomes a variable that the market must guess.
Friedman’s critique does not depend on the quantity theory of money. Keynesian policies do not necessarily follow even if the Keynesian theory of the business cycle were conclusively proved.
It must also be demonstrated that the government has the ability and willingness of the government to act as the theory prescribes. We are therefore further assuming that central banks have the incentive to stabilise the economy. If the government lacks the information required to stabilise the economy, issues of public choice incentives become fully redundant. Incentives to pursue an objective do not matter if the objective itself is unattainable.
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