请教各位前辈和大神们，用SAS做固定效应模型，程序如下：
proc panel data=all;
   id  area1_ year;
   class gender_ age_ education_ marriage_ work health_ visit area2  insurance_ family_ old_age_support_ socialfreq_;
   model total_med_cost=gender_ age_ education_ marriage_ work health_ disability chronic_dis visit income area2  
      insurance_ family_ old_age_support_ socialfreq_/fixtwo;
run;

问题1：area1_是”城乡“二分类变量，报错如下：ERROR: Data set WORK._CLSS_ is not sorted in ascending sequence with respect to time series ID.  The current time period  has year=2011 and the previous time period has year=2015 in cross section area1_=2.   对变量重新排序后还是报错，该问题是因为area1_这个变量不唯一吗？应该如何解决这个问题？
问题2：想对一些分类变量哑变量化，并指定对照组，该如何写程序？