Experimental methods used in the author’s 2003 study of misalignment with opposed angles

The experiments were done with the graphics package Appleworks on a Powerbook with a screen approximately 11 inches by 8. A separate file was made for one handedness of each angle pair, with angle arms just over one inch long on the screen, and test arms at 0° (vertical). The gap between test points was just over two inches. A chart was prepared to assign a number to each of the 18 orientations of each angle pair, (at 10° intervals from -5° through to 165°), and to the 18 mirror orientations of each pair. 144 paper slips were prepared, each bearing a number corresponding to each unique combination of orientation and handedness. These were then selected one by one, by lucky dip. For each test adjustment, the computer file for the relevant angle pair was first called up and copied. In the copy file, using the marquee tool, one or other angle (informally varied) was set out of alignment. The image was then mirror reversed if necessary, and set to rotate to the orientation specified by the selected slip number. A possible problem arose because in Appleworks lines near to cardinal orientations are very jagged, but informal tests suggested that this did not seem to give rise to any "twisted cord" type orientation illusion. The test lines were then adjusted to apparent alignment, using the marquee tool, once again with informal variation in which angle was moved. Following the methods of Weintraub, the distance between test points was not controlled, though the attempt was made to keep it at approximately two inches, on the assumption that angular error remains constant with variation in separation. This avoided the need for distracting position references on the screen, which was also uncluttered because in Appleworks the outlines of the marquee tool vanish whilst selections are moved.  Only a small arrow offered any visual distraction. The file was then printed and closed without saving, and the printed sheets set aside without being inspected. A tick was then put on the numbered slip, and the slip was returned to the lucky dip until it had accumulated three ticks, when it was set aside. Three tests were therefore made for every orientation of each angle and its mirror partner. The experiment was run over several days, with not more than thirty adjustments in a single session. (I find that acute angle judgments are strangely taxing at certain orientations, and suspect that performance may fall off for observers who soldier through longer sequences in one session, however conscientiously). When the tests seemed complete, the errors were measured by hand and tabulated. In tabulating the data, it was found that a few orientations had accidentally been tested too many times and a few tests had been omitted. The absent tests were therefore done as a final sequence, but the extra tests were retained to feed into the average for their orientation, to avoid a decision as to which test result to discard.