reverse_helmert_code: Reverse Helmert code

In contrastable: Consistent Contrast Coding for Factors

Reverse Helmert code

Description

Reverse helmert coding is the same concept as helmert coding, but the order of the groupings is reversed. See also helmert_code.

Usage

reverse_helmert_code(n)

Arguments

n	Integer umber of factor levels to compute contrasts for.

Details

Reverse helmert coding compares each level to the total mean of all levels that come after it. Differs from forward difference coding, which only compares pairs of levels (not a level to a cumulative mean of levels).

Example interpretation for a 4 level factor:

• Intercept = Grand mean (mean of the means of each level)

• grp1 = mean(grp4, grp3, grp2) - grp(1)

• grp2 = mean(grp4, grp3) - mean(grp2)

• grp3 = mean(grp3) - mean(grp4)

Value

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

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 ~ reverse_helmert_code)
lm(resp ~ grp, data = mydf)

contrastable documentation built on Oct. 1, 2024, 5:07 p.m.