Dear all,

I've been having problems selecting the correct covariance structure and for my mixed model (employees nested within firms).
When adding a (categorical) random slope (regarding the size of the firm) to my model, my model doesn't converge with the default (independent) covariance structure and only converges when using the cov(exchangeable) option (for the R. notation, only independent and exchangeable covariance structures are available)
Code:
mixed jobsat level1fixedeffects level2fixedeffects || firmid: R.size
Did I understand it correctly, that one uses the exchangeable covariance structure only for repeated measures/panel/longitudinal data? I'm using pooled cross-sectional data, looking at firms for 2 consecutive years and treating the year variable as a fixed effect, so the observations (employees) on the first level are not the same for both years. I believe the covariance structure should therefore be independent and not exchangeable, so the non-convergence just tells me that the model is not sufficient and I shouldn't use it.

Please tell me if you need further information in order to answer my question regarding the covariance structure.
Your input will be greatly appreciated.

Felicia