ancova.factorial.contrasts: Factorial Contrasts

In pwrss: Statistical Power and Sample Size Calculation Tools

Factorial Contrasts

Description

Helper function to construct the default contrast coefficients for various coding schemes.

Note that R has a partial matching feature which allows you to specify shortened versions of arguments, such as coding instead of coding.scheme.

Validated using lm() and aov() functions.

Usage

factorial.contrasts(factor.levels = c(3, 2),
                    coding.scheme = rep("deviation", length(factor.levels)),
                    base = factor.levels,
                    intercept = FALSE,
                    verbose = TRUE)

Arguments

factor.levels	Integer. Number of levels or groups in each factor. For example, for two factors each having two levels or groups use e.g. c(2, 2), for three factors each having two levels or groups use e.g. c(2, 2, 2)
coding.scheme	Character vector. Coding scheme for each factor. "sum" for deviation or effect coding, "treatment" for dummy coding, "helmert" for Helmert type of coding, and "poly" for polynomial coding. Each factor can have their own coding scheme. If a single character value is provided, it will be copied to other factors
base	Integer vector. Specifies which group is considered the baseline group. Ignored for coding schemes other than "treatment"
intercept	Logical. FALSE by default. If TRUE contrast matrix includes the intercept
verbose	Logical. TRUE by default. If FALSE no output is printed on the console

Value

factor.levels	Number of levels (or groups) in each factor
factor.data	Unique combination of factor levels
model.matrix	Model (design) matrix based on unique combination of factor levels
contrast.matrix	Contrast matrix

Examples

###################################################
############### dummy coding scheme ###############
####################################################

# one factor w/ 3 levels
contrast.object <- factorial.contrasts(factor.levels = 3,
                                       coding = "treatment")
# model matrix
contrast.object$model.matrix

# contrast matrix
contrast.object$contrast.matrix

#######################################################
###############  deviation coding scheme ##############
#######################################################

# especially useful for factorial designs
# two factors w/ 2 and 3 levels, respectively
contrast.object <- factorial.contrasts(factor.levels = c(2, 3),
                                       coding = "sum")

# model matrix
contrast.object$model.matrix

# contrast matrix
contrast.object$contrast.matrix


######################################################
###############  Helmert coding scheme ###############
######################################################

# one factor w/ 3 levels
contrast.object <- factorial.contrasts(factor.levels = 3,
                                       coding = "helmert")

# model matrix
contrast.object$model.matrix

# contrast matrix
contrast.object$contrast.matrix

#########################################################
###############  polynomial coding scheme ###############
#########################################################

# one factor w/ 3 levels
contrast.object <- factorial.contrasts(factor.levels = 3,
                                       coding = "poly")

# model matrix
contrast.object$model.matrix

# contrast matrix
contrast.object$contrast.matrix

pwrss documentation built on Sept. 16, 2025, 9:11 a.m.