Return a matrix of mean difference contrasts. This is an adjustment to the contr.orthonormal that ascertains that the prior distributions on difference between the gran mean and factor level are identical independent of the number of factor levels (which does not hold for the orthonormal contrast). Furthermore, the contrast is re-scaled so the specified prior distribution exactly corresponds to the prior distribution on difference between each factor level and the grand mean -- this is approximately twice the scale of contr.orthonormal.

contr.meandif(n, contrasts = TRUE)



a vector of levels for a factor, or the number of levels


logical indicating whether contrasts should be computed


A matrix with n rows and k columns, with k = n - 1 if contrasts = TRUE and k = n if contrasts = FALSE.


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contr.meandif(c(1, 2))
#>      [,1]
#> [1,]   -1
#> [2,]    1
contr.meandif(c(1, 2, 3))
#>            [,1] [,2]
#> [1,]  0.0000000  1.0
#> [2,] -0.8660254 -0.5
#> [3,]  0.8660254 -0.5