Fast factorial contrast generators — generate_interaction

Returns a matrix N_cells x N_contrasts - each row is a design cell, columns are independent contrasts (difference-coded for the factors you ask for, grand-mean for the rest). Suitable for tcrossprod(dm, C) or lm.fit(design, y) followed by %*% coef in the usual way.

Usage

generate_interaction_contrast(des, factors)

generate_main_effect_contrast(des, factor)

Arguments

des: data.frame with one column per factor (must be factor)
factors: character vector: which factor(s) get difference coding. - generate_main_effect_contrast() takes a single factor name.
- generate_interaction_contrast() takes >= 2 for an interaction (or 1 to reproduce a main-effect matrix).
factor: Single factor name for the main effect.

Value

numeric matrix nrow = prod levels(f) , ncol = prod (Li - 1) for the chosen factors.

Examples

des <- expand.grid(Time = factor(1:4),
                   Cond = factor(c("face","scene")))

# Main effect of Time (4-1 = 3 contrasts)
M <- generate_main_effect_contrast(des, "Time")

# Full TimexCond interaction ( (4-1)*(2-1) = 3 contrasts )
I <- generate_interaction_contrast(des, c("Time","Cond"))
dim(I)   # 8 rows (cells) x 3 columns (contrasts)
#> [1] 8 3