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
1.3	     08/09/14  minor bug fixed
1.4	     04/23/16  minor bug fixed	
1.5	     07/03/16  Implementation of Difference-in-Differences with nearest neighbor matching on pre-treatment outcome variables
1.6	     07/18/17  standard error with degress of freedom correction (two-way)
1.7	     11/25/18  standard error with degress of freedom correction (one-way)
