Hi
I want to know how to use the "noconstant" option in xtunitroot test.

According to Stata manual, "noconstant suppresses the panel-specific mean term in the model that describes the process by which the series is generated. Specifying noconstant imposes the assumption that the series has a mean of zero for all panels."

I cannot find out When should I use the "noconstant" option in xtunitroot test based on Stata manual descriptions.

For example, the "demean" option mitigates the impact of cross-sectional dependence in Stata manual; then what is for "noconstant" option in xtuniroot test?