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Welcome to the 2 Alternative Forced Choice module, a module within the Evaluation Toolkit. The 2 Alternative Forced Choice (2AFC) method
is a psychophysical approach to find a
. In this method, a subject is shown a comparison between two alternatives and forced to make a choice. This can be used to determine a
by, for example, showing comparisons between varying levels of the test stimuli and a baseline stimuli and asking the subject to identify the baseline (or the test). Typcially, in a 2AFC procedure, the baseline will be compared to itself and one should get 50% random chance for that level. Far above the discrimination, everyone should be able to see an effect and one should get 100% of subjects choosing the baseline image. The discrimination threshold is defined as the point at which 75% of respondents see the effect under study.
In this image-based adaptation of the 2AFC method, a subject is shown a series of comparisons or image pairs. Each image pair includes the baseline image and one of the stimuli images which run through the various levels of the stimuli. The 2AFC module automatically randomizes the left/right appearance of the images and also randomizes the order in which the image pairs are shown.
This 2AFC module provides a great deal of flexibility, allowing the user to provide multiple baselines and multiple variants of each stimuli level. This is a useful feature for experimental design. Multiple baselines, each slightly different, ensure that a study subject can't memorize which one is correct. Likewise, multiple variants for each stimuli level allows multiple independent trials for each stimuli level.
In this demo, we will use use example images that vary in value (lightness) to demonstrate the functionality of the 2AFC module. The baseline images have V=0 (black) while the stimuli levels have V=[0, 5, 10, 15, 25, 35, 45]. Each level, both for the baseline and the stimuli, comes in five variations. The variation is indicated by the number on the image from 00 to 04. One of the possible five variants will be shown for the baseline and for the stimuli levels. Note that the baseline is also included in the stimuli levels so that the baseline will be compared to a baseline.
Click on SUBMIT to start the demo study.
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