Poggendorff Experiment

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Introduction
Design
Method
Data Analysis
References


Introduction
In the arrangement of lines below, the diagonal line segments do not appear collinear when in fact they are. This illusion is known as the Poggendorff illusion after its founder.  Several factors are known to affect the degree of misperception experienced in this illusion with one being the separation of the verticals. The greater the separation, the greater the illusion. This experiment allows students to test the effect of separation of the verticals relative to the effect of length of the diagonals.
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Design

The experiment uses a 2 x 3 factorial design that crosses three levels of a separation variable (wide, medium, and narrow) with two levels of a length variable (long and short). Both variables are manipulated within subjects. The order of trials is fixed, beginning with four Narrow-Long trials, followed by four Medium-Long trials, four Wide-Long trials, four Narrow-Short trials, four Medium-Short trials, and four Wide-Short trials. Order is fixed on the assumption that the illusion neither strengthens nor diminishes with time. The dependent variable is the deviation in pixels between the height of the adjusted diagonal line segment (the right diagonal) and the height of a true collinear diagonal line segment. A negative value indicates that the right diagonal was placed too low, which is the placement that reflects the Poggendorff illusion.
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Method

On each trial participants drag the upper right diagonal to the position they perceive to be collinear with the fixed lower left diagonal. The initial position of the upper right diagonal is randomly set to be both above and below the true collinear position of the lower left diagonal.
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Data Analysis 

The factorial design of the experiment invites an analysis to determine the relative importance of diagonal length and vertical line separation on the magnitude of the Poggendorff illusion. The analysis can be performed by comparing differences in the magnitude of the marginal means for Wide, Medium, and Narrow trials on the one hand and Long and Short diagonal trials on the other. Students who have experience with the analysis of variance can use this technique to test the significance of the two possible main effects as well as the effect of the interaction of vertical separation and diagonal line length.
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Data Format for Downloadable csv file
The data obtained via the Download data link on the PsychExperiments homepage are comma delimited.  The first entry (up to the first comma) is the affiliation of the research participant.  Typically this is the class to which the participant belongs.  Non-affiliated participants use the default entry of Interested Person. Next is the randomly assigned ID code for the research participant. The third and fourth entries are the data and time (24-hour format) on the server when the data were received.  Next is a text entry for gender. F is for female and M for male. After gender comes an integer entry for age.  Raw data from the experiment follow age.  The data consist of 24 trials.  The first data entry for each trial is a trial number (1-24).  This is followed by a text entry for value of the length variable (Long or Short) and by a text entry for the separation variable (Narrow, Medium, or Wide). The next entry is the magnitude of the illusion, with negative values indicating that the illusion was experienced. Decimal values to two places are used because decimal value constants are used in computing error.  (When this program is revised Summer of 2002, this potentially confusing calculation will be modified to produce positive integer values for errors rather than the current negative decimal values.) 

 The table below gives the data format (excluding user information) The text line below that shows a complete data set for one research participant.

Trial Number Length Width Magnitude of error (pixels) Trial Number Length Width Magnitude of error (pixels) etc.
1, Long, Narrow, -1.25 2, Long, Narrow, -2.50, etc. 
Loras College--Pusateris PSY 332,RBK564,3/22/2002,18:01,M,20,1,Long,Narrow,-1.25,2,Long,Narrow,-2.50,. . .24,Short,Wide,-3.33,

References

Coren, S.& Girgus, J.S. (1978). Visual Illusions. In R. Held, H. W. Leibowitz, & H.-L. Teuber (Eds.) Handbook of Sensory Physiology VIII: Perception (pp. 549-568), Berlin: Springer.

Coren, S.& Girgus, J.S. (1978). Seeing is deceiving: The psychology of visual illusions. Hilldale, N.J.: Lawrence Erlbaum.
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Last revised:November 01, 2003 07:01:31 PM
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