The registrar at a small college says she can predict the number of units that a full-time student has accumulated on the basis of the number of semesters the student has attended the college by using the equation

Predicted Units = 0 + 14 Semesters

This tells us the deterministic part of the regression model. What factors might contribute to the random component? In other words, why might a student’s number of units not fall exactly on this line?