N <- 21
post_test <- 
c(116.2, 116.9, 106.9, 104.6, 114.2, 113.6, 116.6, 114.8, 114.9,
111, 113.9, 115.6, 116.2, 119.6, 109.6, 122, 109.7, 112.4, 115.5, 119.7, 111.5)
pre_test <- 
c(105.9, 100.8, 91.7, 97.5, 106.5, 107.4, 111.4, 110, 106.9, 106.7,
104.1, 109.7, 110.6, 115.2, 101.9, 119.8, 108.8, 104.6, 111.1, 115.5, 107.2)
grade <- 
c(4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4)
treatment <- 
c(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1)
supp <- 
c(1, 0, 0, 0, 0, 1, 1, 1, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1, 1, 0, 0)