This relates to Aaron’s question from the class quodlibet.
Attributed to Jan L. A. van de Snepscheut:†
In theory, there is no difference between theory and practice. But, in practice, there is.
In a nutshell, this is the problem with teaching macroeconomics. Is there any other field where the theory in texts is more blithely ignored by the practitioners?
† This quote is often mis-attributed to Yogi Berra.