Hooke's law states that the restoring force of a spring is directly proportional to a small displacement. Students in physics class were conducting an experiment to determine the difference in displacement among various springs. They wanted to determine if displacement not only varied with mass but also with the type of spring used. The class tested twenty springs; they varied in length, circumference, and material. They used an assortment of masses as well. On some springs they used masses of 5g increments; on others, masses of 10g increments. After hanging the masses from each spring, they measured how far the springs stretched. The students concluded that thin, aluminum springs had greater displacement than short, thick, iron springs. How could the students improve the validity of the data in this experiment to better support their conclusion?.