Researchers are investigating whether middle school children differ significantly in their critical-thinking ability by measuring the differences between sixth, seventh, and eighth graders. Each student at Tannenbaum Middle School receives the same intelligence test, and then the students’ scores are compared. Researchers also record the age of the students to see if age affects performance on the test. Only participants whose first language is English are used. Identify whether each item should be either a constant or a variable in this particular study.