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There are two independent variables (see figure 1 below): 1) ARROWHEAD variable: the arrowheads on the comparison line will either be the same or different from the arrowheads on the standard line. 2) TYPE-OF-TRIAL variable: at the beginning of a trial, the comparison line will be either longer than the standard line, or it will be shorter. If it is longer, because you will mostly likely reduce the length of the comparison line, this is called a descending type of trial; if it is shorter, because you will mostly likely increase the length of the comparison line, this is called an ascending type of trial.
The dependent variable will be how accurate you are in setting the bottom (comparison) line to match the length of the top (standard) line.