-xtunitroot ips- error for 'no observations' and -trend- [推广有奖]

Hi, i am new to STATA and running an unbalanced panel. i am wondering ask two questions:

1) when i use -xtunitroot ips- to examine the variable's stationarity, some variables display 'no obserevations, e (2000)', i have tried -xtunitroot fisher- and the same problem comes again, what can i do? it seems not beause of lots of missing values in each panel.

2) what is exacatelly meaning of -trend- commonand following up -xtunitroot ips-? becuase the P-value are substainally different between with -trend- and without -trend-.

any help is appreciated.

Pan