
    library(AER)
    data(CPS1985)
    dummy_mod <- lm(wage ~ gender, data = CPS1985)


  # test whether the gender gap is significantly different from zero 
  # use robust standard errors


  coeftest(dummy_mod, vcov. = vcovHC, type = "HC0")
  # or
  linearHypothesis(dummy_mod, "genderfemale=0", vcov. = vcovHC, type = "HC0")


  test_predefined_objects("dummy_mod")
  test_or({
    test_function("coeftest", args = c("x", "vcov.")) 
  },{
    test_function("linearHypothesis", args = c("model", "vcov.", "hypothesis.matrix"))
  },{
    test_function("linearHypothesis", args = c("model", "hypothesis.matrix", "type"))
  })
  success_msg("Right! The hypothesis that there is no wage gap is rejected at any common level.")