
  # attach the package and load the dataset
  
  
  
  # obtain an overview over the dataset
  
  
  # verify that Guns is a balanced panel
  years  <- 
  states <-


  # attach the `AER` package and load the `Guns` dataset
  library(AER)  
  data("Guns")  
    
  # obtain an overview over the dataset
  summary(Guns)
  
  # verify that `Guns` is a balanced panel
  years  <- length(levels(Guns$year))
  states <- length(levels(Guns$state))
  years*states == nrow(Guns)


  ex() %>% check_library("AER")
  test_function("data")
  test_or({
      ex() %>% check_function("summary") %>% check_result() %>% check_equal()
    },{
      f <- ex() %>% override_solution("str(Guns)") 
      f %>% check_function("str") %>% check_result() %>% check_equal()
    })
  ex() %>% check_object("years") %>% check_equal()
  ex() %>% check_object("states") %>% check_equal()
  ex() %>% check_function("nrow") %>% check_arg("x") %>% check_equal()
  test_or({
      test_student_typed("years*states == nrow(Guns)")
    },{
      test_student_typed("states*years == nrow(Guns)")
    },{
      test_student_typed("nrow(Guns) == years*states")
    },{
      test_student_typed("nrow(Guns) == states*years")
    })
  success_msg("Correct! There are 51 states (entities) over observed over 23 years (time periods). The product of the number of entities and equals the number of total observations in the dataset.")