MACRO-ECONOMICS SHOULD NEVER BE DESCRIBED WITH AN EQUATION
It is very tempting for a certain sort of person to want to describe human behavior as a set of defined variables. AX-B=C sort of thing where changing any one of the variables changes the whole equation. The problem is that human actors do not behave in this way. People behave in a much more linear fashion. A leads to B leads to C. See also Cargo Cults.