
      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))


      # generate the subsets
      
      # compute the group means


           # generate the subsets
           dat_NJ <- subset(dat, state == 1)
           dat_PA <- subset(dat, state == 0)
           
           # compute the group means
           dat %>% 
            group_by(state) %>% 
            summarise(mean(FTE, na.rm = T),
            mean(FTE2, na.rm = T))


           ex() %>% check_predefined_objects("dat")
           ex() %>% check_object("dat_NJ") %>% check_equal(undefined_msg = "You have not defined `dat_NJ`!")
           ex() %>% check_object("dat_PA") %>% check_equal(undefined_msg = "You have not defined `dat_NJ`!")
           
           ex() %>% check_or(
             ex() %>% check_output_expr("dat %>% 
              group_by(state) %>% 
              summarise(mean(FTE, na.rm = T),
              mean(FTE2, na.rm = T))", missing_msg = "It seems that not all group means have been correctly printed to the console."),
              ex() %>% check_output_expr("mean(dat_NJ$FTE, na.rm=T);mean(dat_NJ$FTE2, na.rm = T);mean(dat_PA$FTE, na.rm = T);mean(dat_PA$FTE2, na.rm = T)", missing_msg = "It seems that not all group means have been correctly printed to the console.")
            )
            success_msg(msg = "Corect!")