Changes in ssmrob version 0.7
+ Generic functions nobs and model.matrix were added.
+ print and summary outputs were adjusted to mimic more standard R model printing outputs
+ fitted model objects were extended, see documentation
Changes in ssmrob version 0.6
+ Order of arguments in functions ssmrob, heckitrob, heckit5rob, etregrob was
changed. The selection formula is first, the outcome formula(s) is second.
Changes in robmed version 0.5
+ Number of digits after comma in print functions reduced to 4.
+ Chi-square probability for covariate weights is 0.95, it was 0.9.
+ Splitted weight calculation for design space is introduced (see Zhelonkin,
Genton, Ronchetti 2016, p.814).
+ etregrob function is added, robust estimation for Endogenous Treatment Model.