st: gen lag variable

["irodriguez" <irodriguez@utdt.edu>]

I have a panel data for 1200 firms by year (10 years for each firm)

I need to create a lagged variable of one called  roa but it must be lagged for each firm

I tried with

by empresa: egen lagroa= roa[_n-1]

but the command give me this error: r(198)

What can I do?

Thanks