How to fix ascending sequence error?

[ccherrub]

Hi so I keep trying to do a first difference approach and end it with the code:

proc panel Data=MM6 plots=none;
   ID Statefips Year;
   Model ND = treatment / FIXONE noint HCCME=2 Cluster;
run;

However I keep recieving this error:

The data set WORK.MM6 is not sorted in ascending sequence with respect to the time series ID. The current time period has
year=2009 and the previous time period has year=2009 in cross section statefips=1.

How would I go about to fix this? Thanks for the help!

[Patrick]

I believe this forum discussion should provide all the guidance you need.