Modeling & Regression Errors

Contrasts require 2+ factor levels

Matches: contrasts can be applied only to factors with 2 or more levels

WHAT THIS ERROR MEANS

A factor variable in your model has only one level (or zero after removing NAs). R needs at least two levels to create contrast (dummy) variables.

1. Check factor levels with levels() and table().

2. Remove single-level factors from the model.

3. Filter out NA values that may be reducing levels.

BAD — THIS CAUSES THE ERROR

# region has only one unique value after filtering
lm(price ~ region + size, data = df)

GOOD — CORRECT APPROACH

# Remove constant factors
df_clean <- df[df$region %in% names(which(table(df$region) > 0)), ]
lm(price ~ size, data = df_clean)

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