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The experiment has one independent variable: the offset of the comparison line from the true connection point. If the comparison line is below the true connection point, then the offset is negative; if the comparison line is above the true connection point, then the offset is positive. At the start of each trial, the comparison line will be located either at the true connection point (i.e., offset = 0) or at a location either above (i.e., offset > 0 ) or below (i.e., offset < 0). < 0) the true connection point. For example, the picture below shows a comparison line well above the true connection point:
The experiment will have the comparison line displayed at many offsets, both above and below the true connection point. For example, if there are 3 offsets above and 3 offsets below the true connection point, then including offset = 0, the experiment will have 7 offset values; each trial of the experiment will display the comparison line at one of these offsets. The total number of trials depends on how many times you will see a comparison line at each offset. For example, if the experiment requires you to make 5 choices about a comparison line at each offset, and there are 7 offsets, then the total number of trials will be 5 * 7 = 35. This procedure is called the method of constant stimuli.