The child was so tall when she was two, three, four, etc. One could take a piece of graph paper and plot these data points to observe a pattern of that child's physical growth in height. Likewise, if the child's math achievement level had been measured annually from vertically scaled tests, then the child's growth pattern in math could be observed just as for height. Because we follow the progress of each student individually over time and measuring the deflections in patterns, each student in essence is serving as his or her own control which in turn allows for the objective measurement of progress of various groups of students. Beyond this conceptual view, the process is statistically and computationally complex.
Students miss tests.
Teachers work in self-contained classrooms, in teams, in subject matter departments or in combinations of these and other assignment patterns. Some students are retained in grades while others are permitted to skip grades. The tests must have sufficient stretch to measure the progress of both previously low and high scoring students. The tests must demonstrate appropriate reliability. If your students make progress, they will show a positive gain in their scores.