Hi there,

I'm working on a dataset with individual-level and school-level variables using multilevel model. The independent variable is a categorical variable (with four categories) which is transformed to three dummy variables.

Code:
mixed PV1MATH ESCS male i.gr9 i.gr1011 i.c1p_sg1 i.c2p_sg1 i.cc_fg1 ///
immiage ltt ESCSave IMMIrate||CNTSCHID:
That works.

But when I tried to test the random effect of school-level variables on these dummy variables' slops, there were always some errors:

First, the code is like this:
Code:
mixed PV1MATH ESCS male i.gr9 i.gr1011 i.c1p_sg1 i.c2p_sg1 i.cc_fg1 ///
immiage ltt ESCSave IMMIrate||CNTSCHID: i.c1p_sg1 i.c2p_sg1 i.cc_fg1
Then Stata told me that
must use R. when specifying factor variables in random-effects equations
Then I tried:
Code:
    mixed PV1MATH ESCS male i.gr9 i.gr1011 i.c1p_sg1 i.c2p_sg1 i.cc_fg1 ///
    immiage ltt ESCSave IMMIrate||CNTSCHID: R.c1p_sg1 R.c2p_sg1 R.cc_fg1
Stata said:
R.c1p_sg1 R.c2p_sg1 R.cc_fg1 invalid level specification
Then I typed:
Code:
mixed PV1MATH ESCS male i.gr9 i.gr1011 i.c1p_sg1 i.c2p_sg1 i.cc_fg1 ///
    immiage ltt ESCSave IMMIrate||CNTSCHID: c1p_sg1 c2p_sg1 cc_fg1, cov (un)
Stata told me that
Warning: standard-error calculation failed
Then I tried:
Code:
mixed PV1MATH ESCS male i.gr9 i.gr1011 i.c1p_sg1 i.c2p_sg1 i.cc_fg1 ///
    immiage ltt ESCSave IMMIrate||CNTSCHID: c1p_sg1 c2p_sg1 cc_fg1
Stata gives a result but no standard errors:
Random-effects Parameters | Estimate Std. Err. [95% Conf. Interval]
-----------------------------+------------------------------------------------
CNTSCHID: Independent |
var(c1p_sg1) | 549.0446 . . .
var(c2p_sg1) | .00066 . . .
var(cc_fg1) | 3.43e-13 . . .
var(_cons) | 1068.627 . . .
-----------------------------+------------------------------------------------
var(Residual) | 4851.256 . . .
------------------------------------------------------------------------------
LR test vs. linear model: chi2(4) = 601.67 Prob > chi2 = 0.0000

I want to know why that happens and how to deal with these problems.

Also, I have to admit that I don't know much about the principle of how multilevel model deals with dummy variables and I should have understood. Could you please also recommend some books or papers if you know any reference for related issues?

Thank you very much.