Systematic variation is variation due to what?

• A. Random noise
• B. A genuine effect explained by the model
• C. Measurement error
• D. Outliers

Answer: (B)

Systematic variation is the part of the observed variation in the outcome that comes from the real relationship between the variables—the signal the model is trying to capture. In regression or ANOVA, this is the variation explained by the predictors (the genuine effect the model accounts for). It contrasts with random noise, which are unpredictable fluctuations the model cannot explain. Measurement error typically adds to this random, unexplained portion, and outliers are unusual observations that don’t define the normal systematic pattern.