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This applet contains many independent variables, that is, stimulus features that have an effect on the strength of the illusion. For example, the angle of the two tilted lines, the placement of the two horizontal lines, the length of the standard line and the type of trial all have an effect on how strongly the illusion will affect your perception. Only the type of trial will be manipulated during the experiment--the other independent variables will be set through a parameter list on the HTML page that launches the applet.
As described in the procedure section above, half the trials will start out with the comparison line shorter than or equal to the standard line, and your task will be to increase the size of the comparison line until it appears longer than the standard line. This is called an ascending type of trial. The other half of trials will start out with the comparison line appearing either longer or equal to the standard, and your task will be to decrease the size of the comparison line until it appears shorter than the standard line. This is called a descending type of trial.