Hi all,

I have the following dataset where I want to generate a lagged variable.

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input double fyear str58 conm double at
1989 "A.A. IMPORTING CO INC"   10.109
1989 "AAR CORP"               388.521
1990 "AAR CORP"               379.958
1991 "AAR CORP"               395.351
1992 "AAR CORP"               365.151
1993 "AAR CORP"               417.626
1994 "AAR CORP"               425.814
1995 "AAR CORP"               437.846
1996 "AAR CORP"               529.584
1997 "AAR CORP"               670.559
1998 "AAR CORP"                726.63
1999 "AAR CORP"               740.998
2000 "AAR CORP"               701.854
2001 "AAR CORP"               710.199
2002 "AAR CORP"               686.621
2003 "AAR CORP"               709.292
2004 "AAR CORP"                732.23
2005 "AAR CORP"               978.819
2006 "AAR CORP"              1067.633
2007 "AAR CORP"               1362.01
end
The lagged variable I want to create should be the one year lagged variable of "at", e.g. so that in the line of year 1995 for AAR CORP the "at" from year 1994 are shown.

I already tried the following code which I found here on the forum, but I was reported a string problem. Perhaps you can help me in this regard:

Code:
xtset conm fyear, year
string variables not allowed in varlist;
conm is a string variable
r(109);

gen size = l.at
time variable not set
r(111);
Thanks for your replies

Best

Matthias