peticontrast

The peticontrast package provides a simple way to apply sum coding (effect coding) to factors in OLS models.

Sum Coding vs. Treatment Coding

In R, the default is Treatment Coding (contr.treatment), where each level is compared to a reference category (baseline). The intercept represents the mean of that baseline category.

Sum Coding (implemented here as apply_peticontrast) compares each level to the grand mean (trend) of all categories. In this case:

• The intercept represents the overall average.
• The coefficients represent the deviation of each category from that average.

This is particularly useful for seasonal data where you want to see how each period (e.g., Quarter) differs from the yearly trend.

Installation

# Internal use only

Usage

library(peticontrast)
df <- data.frame(Month = factor(c("Jan", "Feb", "Mar", "Apr")))
df <- apply_peticontrast(df, "Month")
summary(lm(Value ~ Month, data = df))