1. State Space Modeling of Time Series
2. The regression model and Kalman filter model for presidential approval included dummy variables for the transition quarter between administrations.
3. Ghosh (1989) and Watson and Engle (1983) use the EM algorithm to maximize the function because the EM equations are calculated to insure positive estimates of the error variances. However, it is a much more cumbersome to estimate the likelihood function using the EM method. Here, I use the generalized maximum likelihood estimator procedures in GAUSS and RATS and did not encounter any problems with negative variances.
4. I checked the residuals for signs of heteroskedasticity and did not detect a problem. Based on some simulations in which I introduced heteroskedastic errors, the effect of heteroskedasticity in the dynamic linear errors-in-variables setup is to inflate the standard errors, but not to introduced bias.