The data file contains three variables: an id variable that identifies each student in the class, the grade_test1 variable that records the student grade for the first test, and the grade_test2 variab...The data file contains three variables: an id variable that identifies each student in the class, the grade_test1 variable that records the student grade for the first test, and the grade_test2 variable that has the grades for the second test. To formalize this slightly, if X i1 is the score that the i-th participant obtained on the first variable, and X i2 is the score that the same person obtained on the second one, then the difference score is:
The data frame chico contains three variables: an id variable that identifies each student in the class, the grade_test1 variable that records the student grade for the first test, and the grade_test2...The data frame chico contains three variables: an id variable that identifies each student in the class, the grade_test1 variable that records the student grade for the first test, and the grade_test2 variable that has the grades for the second test. This formula tells R everything it needs to know: the variable on the left (grade) is the outcome variable, the bracketed term on the right (id) is the id variable, and the other term on the right is the grouping variable (time).
