xtgls 옵션에 가정에 따라 해당 문제를 해결하되, panel 옵션과 corr 옵션을 활용하여 위배되는 가정을 해결하였다.
The situation ended up being solved according to the assumption of the xtgls option, nevertheless the violating assumption was fixed using the panel option and the corr option.