Track C – Chapter 18 Problem Set (ANCOVA)

This problem set practices the core ideas of Analysis of Covariance (ANCOVA):

  • Between-subjects factor: group (Control vs Treatment)

  • Covariate: pretest

  • DV: posttest

You will interpret:

  1. The covariate effect (does pretest predict posttest?)

  2. The adjusted group effect (does Treatment differ after controlling for pretest?)

  3. The homogeneity of regression slopes assumption (pretest × group)

Run the worked solutions

make psych-ch18-problems

Run only the tests

make test-psych-ch18-problems

Files and outputs

The solution script writes:

  • Synthetic datasets: data/synthetic/

  • Summaries + plots: outputs/track_c/

Exercises

Exercise 1 — True adjusted group effect

A real Treatment advantage exists after controlling for pretest. You should see:

  • significant covariate effect

  • significant adjusted group effect

Exercise 2 — Spurious raw difference (pretest imbalance)

Treatment starts higher on the pretest, but there is no true Treatment effect. You should see:

  • raw posttest means differ

  • ANCOVA group effect becomes non-significant after controlling for pretest

Exercise 3 — Slopes assumption violation

The relationship between pretest and posttest differs by group. You should see:

  • significant pretest × group interaction in the slopes test

  • this is a red flag for standard ANCOVA interpretation