backward_difference_code function

Backward difference code

Compares the mean of level k to level k-1. Differs in direction from forward_difference_code , so be careful to pick the right function. See also contr.sdif .

backward_difference_code(n)

Arguments

• n: Integer umber of factor levels to compute contrasts for.

Returns

A contrast matrix with dimensions n rows and (n-1) columns.

Details

Example interpretation for a 4 level factor:

• Intercept = Grand mean (mean of the means of each level)
• grp1 = mean(grp2) - mean(grp1)
• grp2 = mean(grp3) - mean(grp2)
• grp3 = mean(grp4) - mean(grp3)

Examples

mydf <- data.frame(
  grp = gl(4,5),
  resp = c(seq(1, 5), seq(5, 9), seq(10, 14), seq(15, 19))
)

mydf <- set_contrasts(mydf, grp ~ backward_difference_code)

lm(resp ~ grp, data = mydf)