Many magnitude estimation experiments use a control condition in which subjects are asked to perform magnitude estimations of the length of a line. Why are magnitude estimations of line length a control condition? Magnitude estimations of line length have been shown to be proportional to the actual length of the line (Allard, 2001). If you can show that for a group of subjects magnitude estimations increased proportionally with the length of lines, you have established that the subjects do indeed understand the directions they have been given and can assign numbers to their sensations systematically. This way if your results suggest that another sensation is not scalable, it is likely due to poor conscious perception of the sensation because it can’t be because the subjects can’t perform magnitude estimations.