
      library(foreign)
      library(dplyr)
      data_URL <- "https://github.com/mca91/EconometricsWithR/blob/master/data/fastfood.dta?raw=true"
      dat <- read.dta(data_URL)
      dat <- dat %>% 
        mutate(FTE = nmgrs + empft + (0.5 * emppt),
        FTE2 = nmgrs2 + empft2 + (0.5 * emppt2))
      dat_NJ <- subset(dat, state == 1)
      dat_PA <- subset(dat, state == 0)
      reg_dat <- data.frame(
            rbind(
              data.frame(id = dat$sheet, 
                         chain = dat$chain,
                         state = dat$state,
                         empl = dat$FTE,
                         D = 0),
              data.frame(id = dat$sheet,
                         chain = dat$chain,
                         state = dat$state,
                         empl = dat$FTE2,
                         D = 1)))


      # estimate the regression model
      
      # obtain a robust summary


           # estimate the regression model
           emp_mod <- lm(empl ~ D, data = reg_dat, subset = state == 1)

           # obtain a robust summary
           library(AER)
           coeftest(emp_mod, vcov. = vcovHC, type = "HC1")


           ex() %>% check_predefined_objects("reg_dat")
           ex() %>% check_library("AER")
           
           ex() %>% check_object("emp_mod") %>% check_equal(undefined_msg = "Make sure to not remove `emp_mod`!")

           check_or(
             ex() %>%
               check_function(., "lm") %>% {
                 check_arg(., "formula") %>% check_equal(incorrect_msg = "Looks like the `formula` argument in `lm()` is wrong.")
                 check_arg(., "data") %>% check_equal(incorrect_msg = "Looks like the `data` argument in `lm()` is wrong.")
               },
             ex() %>%
             override_solution("emp_mod <- lm(dat_NJ$empl ~ dat_NJ$D)") %>% 
             check_function("lm") %>% check_arg("formula") %>% check_equal(incorrect_msg = "Looks like the `data` argument in `lm()` is wrong.")
             )

           check_or(
             ex() %>% check_function("coeftest") %>% {
               check_arg(., "x") %>% check_equal()
               check_arg(., "vcov.") %>% check_equal()
             },
             ex() %>% 
             override_solution("emp_mod <- lm(dat_NJ$empl ~ dat_NJ$D); coeftest(emp_mod, vcov. = vcovHC, type = 'HC1')") %>%
             check_function("coeftest") %>% {
               check_arg(., "x") %>% check_equal()
               check_arg(., "vcov.") %>% check_equal()
             }
           )
           success_msg(msg = "Nicely done! The estimate of the coefficient on `D` as well as the t-statistic and thus also the p-value match the results reported by `t.test()`. We cannot reject the hypothesis that the coefficient on `D` is zero.")