sum_code function

Sum code

Wrapper around contr.sum , but ensures that the reference level is the first level alphabetically, not the last. Returns a contrast matrix where comparisons give differences between comparison levels and the grand mean.

sum_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

For n levels of factors, generate a matrix with n-1 comparisons where:

• Reference level = -1
• Comparison level = 1
• All others = 0

Example interpretation for a 4 level factor:

• Intercept = Grand mean (mean of the means of each level)
• grp2 = grp2 - mean(grp4, grp3, grp2, grp1)
• grp3 = grp3 - mean(grp4, grp3, grp2, grp1)
• grp4 = grp4 - mean(grp4, grp3, grp2, grp1)

Note that when n = 2, the coefficient estimate is half of the difference between the two levels. But, this coincidence does not hold when the number of levels is greater than 2.

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 ~ sum_code)

lm(resp ~ grp, data = mydf)