
      library(foreign)
      library(dplyr)
      data_URL <- "https://github.com/Jeshua-Romero-Guadarrama/Econoalgoritmia/tree/main/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)))
      library(AER)
      t_test <- t.test(dat_NJ$FTE, dat_NJ$FTE2, var.equal = F)
      emp_mod <- lm(empl ~ D, data = reg_dat, subset = state == 1)


      # comprobar que las estimaciones y el estadístico t coinciden
      t_test <- t.test(dat_NJ$FTE, dat_NJ$FTE2, var.equal = F)
      emp_mod <- lm(empl ~ D, data = reg_dat, subset = state == 1)
      
      # estimaciones
      emp_mod$coef[2]
      diff(t_test$estimate)

      # estadístico t
      t_test$statistic
      coeftest(emp_mod, vcov. = vcovHC, type = "HC1")[2,3]