In the Muller-Lyer illusion, the two parallel line segments have the same length, but appear to have different lengths because of short angled lines at their ends. A perceptual psychologist wants to know whether the illusion also exists when the lines are shown in white on a black background. Each participant views a pair of lines like the one above (except white-on-black) and uses a knob to adjust the top line so it appears to have the same length as the bottom line. For each participant, the computer records how much shorter the top line is than the bottom line when that participant decides that they appear to match. The best way to assess whether these data constitute evidence of an effect would be