Compound Symmetry Covariance structure

Hi,

I have a question about the random effects model (Specifically, a random
intercept model) in its role in assuming a covariance structure in
estimation. In a panel data textbook, I read that by estimating a random
effects model itself, there is an induced covariance structure.

In nlme package, there are several types of covariance structures such as
Compound Symmetry (which I assume in my model) but the default value is 0.
I initialize it and proceed with the estimation.

Does this mean that if I do not specify the compound symmetry value in
nlme, the estimation is without a covariance assumption or there is
something I have missed in my understanding? That the " by estimating a
random effects model itself, there is an induced covariance structure"
confuses me a little.

It would be very helpful to get an explanation on this.

Thank you very much!

Regards,
Yashree

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