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) 1.8 01/09/19 minor update using cbind() and rbind() instead of cBind() and rBind() 1.9.1 02/13/19 White statistics degress of freedom correction and cleanup