Identification in Linear Simultaneous Equations Models with Covariance Restrictions: An Instrumental Variables Interpretation
在线性联立方程模型中引入协方差约束，推导出识别性的充要条件，并用工具变量解释，为存在协方差约束时的估计提供理论基础。
This picture is greatly complicated when restrictions on the structural disturbance variances and covariances, (henceforth "covariance restrictions") are allowed.Koopmans Rubin, and Leipnik (1950) recognized the usefulness of such restrictions for identification and demonstrated their equivalence to bilinear restrictions on the coefficients.This work was pursued by Wegge (1965) j Rothenberg (1971), and especially Fisher (1963, 1965), surveyed in Fisher (19-66, Chapters 3 and 4).Structural estimation is also complicated by covariance restrictions: as pointed out by Rothenberg and Leenders (1964), system instrumental variables estimators (3SLS) are asymptotically inefficient when covariance i restrictions are present.Two features of these results are (i) the absence of useful necessary and sufficient conditions for identifiability in the presence of covariance restrictions, and (ii) the disappearance of the link between restrictions required for identification and instrumental variables required for estimation.The problem of incorporating covariance restric- tions Into the theory of identification and estimation is thus Incomplete.In this and a companion paper on estimation (Hausman-Taylor (1981)), we provide a simple, complete, and useful solution to the problem In terms of instrumental variables .In the present paper, we derive necessary and sufficient conditions for identifiability in linear simultaneous equations models subject to linear restrictions on the coefficients and covariances.The result, in practical