A study attempted to determine how much money university students spend per year on textbooks and to compare students by year of study. Random samples of 50 students of each of the four groups, first, second, third and fourth year,were surveyed and asked how much they spent on textbooks during the current semester.
(a) Because 3rd and 4th years take higher-level units, which might use more expensive textbooks, we want to compare the means of 1st and 2nd years with the means of 3rd and 4th years. Write a contrast that expresses this comparison.
(b) Write a contrast for comparing 1st and 2nd years.
(c) Write a contrast for comparing 3rd and 4th years.
(d) Are the three defined contrasts orthogonal? Explain.