Appendix B: Accuracy and Precision
Measurements
Gaze accuracy and precision are typically measured in degrees of visual angle. One degree accuracy corresponds to an average
error of 12 mm (0.47
’’
) on a screen at a distance of 65 cm (27
’’
). Data is presented as monocular or binocular. Monocular data
is based on data from the subject
’
s dominant eye presented as monocular or binocular. Monocular data is based on data from
the subject
’
s dominant eye only. Binocular data is the average of both eyes.
B:1 Gaze Precision
Precision measurements are done using artificial eyes to eliminate artifacts from human eye movements. Tobii specifies precision
both with and without noise reduction filters. All measurements are done at the default sampling rate of the eye tracker and at
the optimum distance between the eye tracker and the subject. Precision is calculated as root-mean-square (RMS) of
successive samples.
As the distance from the eye tracker influences precision, measurements are also taken at various distances. Data from these
measurements is binocular.
B:2 Gaze Accuracy
Accuracy under ideal conditions is measured in the center of the head movement box with the subject fixed in a chinrest. Data is
collected immediately after calibration, in a controlled laboratory environment with constant illumination, with 9 stimuli points at
gaze angles
≤
18º. Measurements are done on 20 test subjects without lenses, glasses or droopy eyelids. Accuracy for one
subject is calculated as the mean of several data samples for several stimuli points across a screen. The accuracy figure
presented is the mean accuracy from all subjects.
Good accuracy is difficult to achieve at large gaze angles, but is important when testing large stimuli. For instance, the upper
corner of a 23
’’
screen with the test subject at a distance of 65 cm (26
’’
) from the eye tracker corresponds to a 32º visual angle
from the center of the eye tracker unit. Consequently, measurements are also presented for stimuli presentations at large gaze
angles.
When luminance of the stimuli or the illumination in the lab changes, the size and shape of the pupil is affected. Unless
compensated for, this may cause a significantly reduced accuracy. Testing the influence of surrounding light and stimuli
luminance on accuracy is done in a laboratory environment with controlled light conditions. Stimuli points are presented on a
black background so as not to influence ambient light conditions. Testing is also done with ambient light at a level deemed
‘
normal
’
office lighting where the background is changed to white with black stimuli points. The results from a test in which the
background was changed has been referred to as Drift in previous Tobii product description documents.
As with precision, the distance from the eye tracker influences accuracy. When testing this influence, calibration is done with the
subject in the center of the head movement box (i.e. at a distance of 65 cm) and measurements are done thereafter.
Measurements are performed with the test subject at precise and specific distances relative to the eye tracker, measured along
the axis of the tracking sensors. Data from these measurements is binocular.
12
B: Accuracy and Precision Measurements
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