The Visual Analogue Scale (VAS) is a self-report device that measures the magnitude of internal states such as pain and mood. Conventionally, the VAS is a 100-mm line that is either vertically or horizontally oriented with anchors placed at both poles. Participants place a mark somewhere along the line that best indicates the magnitude of their internal state. Though the reliability and validity of the VAS has been demonstrated, measurement error associated with each style of presentation (horizontal or vertical) still exists. The purpose of this study was to determine whether the measurement error associated with either style of presentation could be accounted for by existing perceptual biases. One hundred and six participants were instructed to make a mark (i.e., point estimate) that divided 9 vertically oriented and 9 horizontally oriented lines according to 9 predetermined lengths. Estimates of perceived line segment length were then derived by subtracting the obtained point estimates. The results of this study suggest that at higher magnitudes of internal state participants were likely to underestimate predetermined lengths using the vertically oriented VAS. The present findings suggest that the law of visual angle could account for the systematic bias associated with the vertically oriented VAS.

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