 That is, one can ask how a change in some exogenous variable in year "t" affects endogenous variables in year "t", in year "t+1", in year "t+2", and so forth.A graph showing the impact on some endogenous variable, over time (that is, the multipliers for times "t", "t+1", "t+2", etcetera), is called an impulse-response function.The general method for calculating impulse response functions is sometimes called comparative dynamics.