Measurement error models with uncertainty about the error variance 1 Running head : MEASUREMENT ERROR MODELS WITH UNCERTAINTY ABOUT THE ERROR VARIANCE

It is well known that measurement error in observable variables induces bias in estimates in standard regression analysis and that structural equation models (SEM) are a typical solution to this problem. Often, multiple indicator equations are subsumed as part of the SEM model – allowing for consistent estimation of the relevant regression parameters. In many instances, however, embedding the measurement model into SEM is not possible because the model would not be identified. To correct for measurement error one has no other recourse than to provide the exact values of the variances of the measurement error terms of the model, although in practice such variances cannot be ascertained exactly but only estimated from an independent study. The usual approach so far has been to treat the estimated values of error variances as if they were known exact population values in the subsequent SEM analysis. In this paper we show that fixing measurement error variance estimates as if they were true values can make the reported standard errors of the structural parameters of the model smaller than they should be. Inferences about the parameters of interest will be incorrect if the estimated nature of the variances is not taken into account. For general SEM, we derive an explicit expression that provides the terms to be added to the standard errors provided by the standard SEM software that treats the estimated variances as exact population values. Interestingly, we find there is a differential impact of the corrections to be added to the standard errors depending on which parameter of the model is estimated. The theoretical results are illustrated with simulations and also with empirical data on a typical SEM model.