The graph below plots the proportion of trials you chose the vertical line without wings as bigger than the line with wings. The line with wings was always 100 pixels long. The x-axis gives the pixel length of the line without wings. The Muller-Lyer illusion is that the line with wings appears to be longer than a line without wings. Typical data shows that proportions are close to zero on the left and increase toward one on the far right. If there was no illusion, the line would be at approximately 0.5 for a line without wings length of 100 pixels. Evidence of the illusory effect would be when the 0.5 proportion is for a pixel length greater than 100 pixels.
If the effect exists, it may be stronger for the group or global averages than for your individual data.
