for_john.R

library(tidyverse); library(lfe)

some_data <- expand.grid(time = 1:10, individual = 1:200) %>% 
  left_join(data_frame(time = 1:10, time_effects=  rnorm(10))) %>% 
  left_join(data_frame(individual = 1:200, individual_effects=  rnorm(200))) %>% 
  mutate(treatment = sample(0:1, n(), replace = T),
         outcome = time_effects + individual_effects + 1 * treatment + rnorm(n())) %>% 
  group_by(time) %>% 
  mutate(demeaned_outcome = outcome - mean(outcome),
         demeaned_treatment = treatment - mean(treatment)) %>% 
  group_by(individual) %>% 
  mutate(demeaned_outcome = outcome - mean(outcome),
         demeaned_treatment = treatment - mean(treatment))


fit_levels <- felm(outcome ~ treatment | time + individual | 0 | 0, data = some_data)

fit_demeaned <- felm(demeaned_outcome ~ demeaned_treatment | time + individual | 0 | 0, data = some_data)

coef(fit_levels) -  coef(fit_demeaned)
vcov(fit_levels) - vcov(fit_demeaned)
summary(fit_levels)$rdf == summary(fit_demeaned)$rdf