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Use backward difference coding

Source: R/contrasts_successive_differences.R
backward_difference_code.Rd

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)

Arguments

n

Integer umber of factor levels to compute contrasts for.

Value

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)
#> 
#> Call:
#> lm(formula = resp ~ grp, data = mydf)
#> 
#> Coefficients:
#> (Intercept)       grp2-1       grp3-2       grp4-3  
#>        9.75         4.00         5.00         5.00  
#>