The relationship between student performance on a state-mandated test administered in the fourth grade and again in the eighth grade has been analyzed for a large group of students in the state. Ellen obtains a score of 540 on the fourth-grade test. From this, her performance on the eighth-grade test is predicted (using the regression line) to be 550.
(a) In what sense can the value 550 be considered an estimated mean?
(b) Why is it an estimated rather than an actual mean?