[Nick Cox]

The same question has the same answer. There is a bug in the command you are using. Sorry, but I don’t have anything to add to what I have said more than once in this thread. If you’re hoping or expecting that I will fix this, that is not going to happen.

Someone who uses xt commands may have a much better answe.

[Daniel Feenberg]

Stata is not complaining about the "sqrt(0)". It is complaining about "*sqrt(0)". That is, the "*" is supposed to separate two arguments, but in the trace output there is only one argument to the * operater. This happens because"`r(rho)'" has evaluated to blank. Could this be because the correlation is undefined? I can't think of any way for that to happen other than each pair of data points having at least one missing value. You could write to the author of the program, or try putting some diagnostic code in to the xtcd program itself.