The understanding of the aggregate fluctuations is a central goal of macroeconomics. Every economy is perturbed by disturbances of various types and sizes at more or less random intervals and those disturbances then propagate through the economy. Macroeconomic schools differ in their hypotheses concerning these shocks and propagation mechanisms. Whether there are on supply or on demand side. Whether there are symmetric or asymmetric information. Whether there is neutrality or illusion of money and all that whether it is in short or long run.