For a project with longitudinal data, I've been trying to read in data from a two wave study to fit some latent growth models. However, for some reason when I run a basic analysis, all the means and covariances look nearly identical and I get a singular covariance matrix. When I run descriptives in SPSS, the means and variances look normal.
In short what I've done: I first imputed plausible values in an R package using IRT for the variable of interest. In SPSS I have transformed the two wave data to age based variables so that I have a variable for each whole age in the dataset. The means of these age based variables look normal in SPSS. When I convert the file to csv/.dat and open te file in Mplus, the means, covariances and variances become as I described before.
Do you have any idea what might be the problem that causes the change in means and covariances/variances?