In the second place, a system would involve more variables than those which may be supposed to enter into demand relationships, and so the preliminary work of estimating the necessary time series would also be increased. In the third place, in the simultaneous method of estimation the estimates of the original parameters come to depend on most of the remaining estimates, and so it is quite likely that they will have large sampling variances.

Studies in the National Income and Expenditure of the United Kingdom, 1954