n	ms	p	gnrp	dp	pp	month	year
1	3.15	2.98	498	1	1	Sep	1999
2	2.52	2.19	510	0	0	Oct	1999
3	2.64	2.75	422	1	1	Nov	1999
4	2.56	2.23	566	0	1	Dec	1999
5	2.98	2.59	536	1	0	Jan	2000
6	2.42	2.26	585	0	0	Feb	2000
7	2.95	2.03	211	1	1	Mar	2000
8	2.76	2.17	374	1	1	Apr	2000
9	2.45	2.28	475	0	0	May	2000

replace month = "Sep-1999" in 1
replace month = "Oct-1999" in 2
replace month = "Nov-1999" in 3
replace month = "Dec-1999" in 4
replace month = "Jan-2000" in 5
replace month = "Feb-2000" in 6
replace month = "Mar-2000" in 7
replace month = "Apr-2000" in 8
replace month = "May-2000" in 9

. tsset month, monthly
varlist:  month:  string variable not allowed
. estat dwatson
time variable not set, use -tsset varname ...-
r(111);

请问，各位高手，在stata中该如何进行DW检测？