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### How The Main Results Are Calculated

(You may want to print this page in order to refer to it after you are finished with the experiment).  At the end of the experiment, the applet will measure the effect of the illusion in each of the four experimental conditions. There are many ways one could measure such an effect. One way is the following:

• 1st) calculate the absolute difference between the length of the standard and comparison line

• (i.e., | Standard - Comparison | )

• 2nd) divide this difference by the length of the standard line (i.e., | Standard - Comparison | / Standard), and

• 3rd) multiply the result by 100 to convert from proportion to percent.

What this gives us is a measure of the effect of the illusion under a certain set of conditions. For example, let's say the standard line was 200 units throughout the experiment. And let's say that at the end of an "ascending, different arrowhead" type of trial you set the comparison line to be 190 units: while the comparison and standard line appeared to be the same length, in reality, the comparison line was 10 units shorter than the standard. We calculate the effect by

• 1st) taking the absolute value of the difference (i.e., | 200 - 190 | = 10)

• 2nd) dividing this difference by the length of the standard (i.e., 10 / 200 = 0.05), and

• 3rd), multiplying this proportion by 100 to get 5%.

In other words, the effect of this type of trial on your perception of relative length was such that you overestimated how long the comparison line was relative to the standard by 5% of the length of the standard.

After you are finished, the first results window (click here) will appear.