When a linear structural equation model results in nonlinear procedures for solution, multiple (but finite) sets of solutions may be possible, even with population data. An example of a three-variable feedback system is given in which two sets of solutions may be generated, and each may be obtained by algebraic analysis or by numerical analysis. Issues of model specification and equilibrium must be considered to differentiate the sets of solutions and choose between them. In the absence of strong information concerning these issues, data at more than one point in time is a minimal requirement to solve for model parameters and evaluate the system of variable relations. Statistical estimation of model parameters is also considered in this context.nterest in and applications of structural equation models has expanded dramatically in the past several years (see, e.g., reviews by Bielby and Hauser, 1978; Cappella, forthcoming; and Fink, forthcoming). Most recently, careful attention has been paid to issues of identification and statistical estimation in such models. The present paper is concerned with the existence of multiple solutions to such models, in the absence of sampling error. Two modeling issues are relevant here: first, the fact that the equations in these models are linear, and second, the notion of identification of model parameters and models as a whole. Some subtle notions are involved here. First, it should be noted that a linear equation system often implies nonlinear constraints on parameters. For example, a system with indirect causal effects will result in estimation in which the products of structural coefficients are constrained to be equal to reduced form coefficients (as in two-stage least squares estimation in multiple regression); a system with unobserved variables constrains the product of structural coefficients to be equal to a covariance or correlation(as at PENNSYLVANIA STATE UNIV on March 15, 2015 smr.sagepub.com Downloaded from