reverse_helmert_code function

Reverse Helmert code

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

reverse_helmert_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

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)

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)