Christopher Sims proposed the Vector Autoregression which is a multivariate linear time series model in which the endogenous variables in the system are functions of the lagged values of all endogenous variables. This allows for a simple and flexible alternative to the traditional structural system of equations. A VAR could model macroeconomic data informatively, without imposing very strong restrictions or relationships. Essentially, it is macroeconomic modeling without much of the a-priori expectations getting in the way.

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