The study of macroeconomics is organized around three models at different time periods, list the three models and distinguish among them

The study of macroeconomics is indeed organized around three main models that correspond to different time periods. These models are:

1. The Classical Model: This model is based on the long-run period. It assumes that the economy is always at full employment or will return to it quickly if it deviates from this level. It also assumes that prices and wages are flexible, and that competition will drive the economy towards full employment. The classical model is often associated with laissez-faire economic policies.

2. The Keynesian Model: This model is based on the short-run period. It was developed by John Maynard Keynes during the Great Depression, a time when the classical model seemed unable to explain the persistent unemployment and economic stagnation. The Keynesian model assumes that prices and wages are sticky, meaning they do not adjust quickly to changes in the economy. Therefore, the economy can be stuck below full employment for a significant period of time. The model suggests that government intervention can help the economy return to full employment.

3. The Monetarist Model: This model is a modification of the classical model and it focuses on the medium-run period. Monetarists, like Milton Friedman, believe that changes in the money supply are the main cause of changes in national output and the price level. They argue that the economy is inherently stable and that government intervention often does more harm than good.

Each of these models offers a different perspective on economic behavior and policy, and they are used to analyze different types of economic problems.