version      date  description
0.1    	 12/12/11  The first version on CRAN
1.0	     07/17/12  First-difference and difference-in-differences estimator added			
1.1	     11/14/12  1) Standard errors with degrees of freedom adjustment, 2) missing data issue fixed, 3) issues with pdata.frame resolved
1.2	     02/05/13  store.wdm option added