Fixed spelling and grammar errors: Corrected mistakes throughout chapters 16 to 21.
Fixed broken links: Updated or removed broken links within chapters 16 to 21.
Updated test in Chapter 19 Exercises: Modified Test 4 to include different possible thresholds for students.
Reorganized Chapter 19 content: Moved the section "Going Deeper with Modelling" to "Base R Linear Model."
Fixed spelling and grammar errors: Corrected mistakes throughout chapters 16 to 21. Fixed broken links: Updated or removed broken links within chapters 16 to 21. Updated test in Chapter 19 Exercises: Modified Test 4 to include different possible thresholds for students. Reorganized Chapter 19 content: Moved the section "Going Deeper with Modelling" to "Base R Linear Model."