contrastable (version 1.0.2)

backward_difference_code: Backward difference code

Description

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.

Usage

backward_difference_code(n)

Value

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

Arguments

n

Integer umber of factor levels to compute contrasts for.

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)