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Use reverse helmert coding

Source: R/contrasts_helmert.R
reverse_helmert_code.Rd

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.

Value

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)
#> 
#> Call:
#> lm(formula = resp ~ grp, data = mydf)
#> 
#> Coefficients:
#> (Intercept)        grp>1        grp>2        grp>3  
#>        9.75        -9.00        -7.50        -5.00  
#>